I am looking for an output amplifier for a signal generator

[mmm]

I trying to build a signal generator with 20 Vpp open circuit , 10Vpp on
50 Ohm output and frequency range up to 10MHz , so I need at least 30MHz
bandwith ( don't blame me if you think is a bit low !! )

I have found almost suitable parts in Analog Device AD815 or Texas
BUF634 plus high speep Op Amp.

Now I am looking for alternative implementation of output stage

An discrete or Op Amp + discrete output stage solution will be very
good if they have a small component count and if the components are easy
to find.


thanks in advance

 

[john jardine]

I trying to build a signal generator with 20 Vpp open circuit , 10Vpp on
50 Ohm output and frequency range up to 10MHz , so I need at least 30MHz
bandwith ( don't blame me if you think is a bit low !! )

I have found almost suitable parts in Analog Device AD815 or Texas
BUF634 plus high speep Op Amp.

Now I am looking for alternative implementation of output stage

An discrete or Op Amp + discrete output stage solution will be very
good if they have a small component count and if the components are easy
to find.


thanks in advance

Being fascinated by function generator type output stages, I looked in on
the AD815 and was going to ask what the problem was, as it seemed perfect
for the job.
First few pages showed a big output swing, big current and massive
bandwidth. It was only until page 5 of the datasheet that the power
bandwidth limitation was revealed. Looks like you caught that as well!.

Discrete output stages tend to end up with lots and lots of trannies and
resistors. A cheap cheap x18 10MHz 40Vpp 50ohm I did a while ago, tots up
11 npns, 11 pnps and 40 R's.

regards
john

 

[Joerg]

Hello John,

Discrete output stages tend to end up with lots and lots of trannies and
resistors. A cheap cheap x18 10MHz 40Vpp 50ohm I did a while ago, tots up
11 npns, 11 pnps and 40 R's.

Maybe a tube? Just kidding. But there is a way to cheat here. You can
still get buffers similar to the HA-0002 and 0033. I have sometimes used
those in a "gang pack" and they'd drive almost anything. For really
hardcore driver needs I stashed away a box of 0063 before they went,
mainly because of their sturdy TO-3 design. Even in a non-feedback
scenario you can achieve output impedances in the low single digits.

Even though I hate sockets, in a function generator it might be a good
policy to socket this stuff and keep a few spares inside dummy sockets.
Just in case someone accidentally transmits into the output. Not that
I'd ever do that, of course, ahem ...

Regards, Joerg

 

[Pooh Bear]

Way back, I'd drooled over using one of National's LH0033s. Most probably
because they'd cheekily advertised it as "Damned fast". Never got the chance
as at the time they cost way too much.
Nowadays if a customer gets wind that I might be inclined to stick a high
performance (=cost!) chip in their kit, they without fail, somehow
miraculously discover, that they can back-off a bit on the spec' of what
they want.
A number of companies I'm now dealing with have been badly burned over the
past 3 years by obsoleting speciality ICs. To the extent that the scanning
radars of company buyers can lock-in on recognition of the usual words such
as cheap, plentiful, rebate, low-cost, but now also 'discrete transistor',
'324' etc!. Strange world.
One day maybe, I'll have the pleasure of using something nice ...

But surely making jellybeans sing should provide any required amount of personal
satisfaction ? In the professional field that is.... ;-)

Graham

 

[John Larkin]

I trying to build a signal generator with 20 Vpp open circuit , 10Vpp on
50 Ohm output and frequency range up to 10MHz , so I need at least 30MHz
bandwith ( don't blame me if you think is a bit low !! )

I have found almost suitable parts in Analog Device AD815 or Texas
BUF634 plus high speep Op Amp.

Now I am looking for alternative implementation of output stage

An discrete or Op Amp + discrete output stage solution will be very
good if they have a small component count and if the components are easy
to find.


thanks in advance

Why not, say, 4 opamp followers, each with a 200 ohm resistor to the
output?

John

 

[john jardine]

Hello John,


Maybe a tube? Just kidding. But there is a way to cheat here. You can
still get buffers similar to the HA-0002 and 0033. I have sometimes used
those in a "gang pack" and they'd drive almost anything. For really
hardcore driver needs I stashed away a box of 0063 before they went,
mainly because of their sturdy TO-3 design. Even in a non-feedback
scenario you can achieve output impedances in the low single digits.

Even though I hate sockets, in a function generator it might be a good
policy to socket this stuff and keep a few spares inside dummy sockets.
Just in case someone accidentally transmits into the output. Not that
I'd ever do that, of course, ahem ...

Regards, Joerg

Way back, I'd drooled over using one of National's LH0033s. Most probably
because they'd cheekily advertised it as "Damned fast". Never got the chance
as at the time they cost way too much.
Nowadays if a customer gets wind that I might be inclined to stick a high
performance (=cost!) chip in their kit, they without fail, somehow
miraculously discover, that they can back-off a bit on the spec' of what
they want.
A number of companies I'm now dealing with have been badly burned over the
past 3 years by obsoleting speciality ICs. To the extent that the scanning
radars of company buyers can lock-in on recognition of the usual words such
as cheap, plentiful, rebate, low-cost, but now also 'discrete transistor',
'324' etc!. Strange world.
One day maybe, I'll have the pleasure of using something nice ...
regards
john

 

[john jardine]

[clip]

But surely making jellybeans sing should provide any required amount of personal
satisfaction ? In the professional field that is.... ;-)

Graham

Yes indeed. You well understand this aspect. There's few finer pleasures
than doodling with a few transistors and other components, looking for an
arrangement that offers up particular properties.
Don't know about the other guys out there but as time goes on, I'm getting
to be more and more of a lazy bastard and won't reinvent a wheel if I can
recycle some tried and tested stuff from a year previous.
But then the fun aspect is lessened and I may as well have just stuck a
fancy chip in, saving tailoring time, hence a quicker path through to the
good stuff, ie those areas needing a bout of idle doodling, (real design?

regards
john

 

[Daniel A. Thomas]

| I trying to build a signal generator with 20 Vpp open circuit , 10Vpp on
| 50 Ohm output and frequency range up to 10MHz , so I need at least 30MHz
| bandwith ( don't blame me if you think is a bit low !! )
|
| I have found almost suitable parts in Analog Device AD815 or Texas
| BUF634 plus high speep Op Amp.
|
| Now I am looking for alternative implementation of output stage
|
| An discrete or Op Amp + discrete output stage solution will be very
| good if they have a small component count and if the components are easy
| to find.
|
|
| thanks in advance

Might want to look at the "Ultra.Fast - Fed Forward Current Booster"
described on page 3 of Linear Technology's AN18 Power Gain Stages for
Monolithic
Amplifiers.

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1100,P1169,D4114

Dan Thomas

 

[Ban]

I trying to build a signal generator with 20 Vpp open circuit , 10Vpp
on 50 Ohm output and frequency range up to 10MHz , so I need at least
30MHz bandwith ( don't blame me if you think is a bit low !! )

I have found almost suitable parts in Analog Device AD815 or Texas
BUF634 plus high speep Op Amp.

Now I am looking for alternative implementation of output stage

An discrete or Op Amp + discrete output stage solution will be very
good if they have a small component count and if the components are
easy to find.

Here is a discrete O/P stage, you can parallel more transistors. I also have
a gain stage somewhere, but do not remember the file name. :-(

+---+-----------+----+-----
| | | |
| | | |
current sources (I) (I) | |
| | _ |/ |/
| +------U-------| BF720
| | bead |> |>
| | | |
| | | |
o +---)---)----+ | |
| |/ | \| | .-. .-.
+-| | |- + | | | |
| |> | <| | | | | |
| +-+-)---+ | '-' '-'
| | | BF720 | | |
| | | | | |
| | | +------+----+-o
| | | BF721 | | |
| +-)-+---+ | .-. .-.
| |< | >| | | | | |
+-| | |- + | | | |
|\ | /| | '-' '-'
+-)-----)----+ | |
| | | |
| | _ |< |<
| +------U-------| BF721
| | bead |\ |\
current sources (I) (I) | |
| | | |
| | | |
+-----+-----------+----+----
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
view\fixed font

 

[Winfield Hill]

Ban wrote...

Here is a discrete O/P stage, you can parallel more transistors.
I also have a gain stage somewhere, but do not remember the file
name. :-(

---+---+-----------+----+-----
| | | |
current sources (I) (I) | |
| | _ |/ |/
| +------U-------| BF720
| | bead |> |>
| | | |
,---)---)----, | |
|/ | \| | .-. .-.
+--| | |--+ | | | |
| |> | <| | | | | |
| '-+-)---' | '-' '-'
| | | BF720 | | |
o---+ | | +------+----+---o
| | | BF721 | | |
| ,-)-+---, | .-. .-.
| |< | >| | | | | |
+--| | |--+ | | | |
|\ | /| | '-' '-'
'-)-----)----' | |
| | | |
| | _ |< |<
| +------U-------| BF721
| | bead |\ |\
current sources (I) (I) | |
| | | |
---+-----+-----------+----+----

The BF720 and BF721 parts come in an attractive SOT-223 package,
but they're 300V parts, which means their beta and gain-bandwidth
are compromised in favor of their high-voltage capability. Other
good choices in the sot-223 package are the fzt2222a, fzt2907a
and pzt4401, pzt4403. These BJTs have f_T in excess of 250MHz,
and are rated at about 1W max each, with 1cm^2 area on the PCB.

 

[Ban]

Ban wrote...

The BF720 and BF721 parts come in an attractive SOT-223 package,
but they're 300V parts, which means their beta and gain-bandwidth
are compromised in favor of their high-voltage capability. Other
good choices in the sot-223 package are the fzt2222a, fzt2907a
and pzt4401, pzt4403. These BJTs have f_T in excess of 250MHz,
and are rated at about 1W max each, with 1cm^2 area on the PCB.

The buffer has the advantage that all 3 terminals of the input transistors
are at the same AC-potential, because the collectors go to the output, thus
a really wide bandwidth can be achieved. I agree with Win, those are
high-voltage parts good for +/-150V not +/-10V. Siemens also makes some dual
RF NPN and PNP with high bandwidth, but I do not know if the supply voltage
is sufficient.

 

[Winfield Hill]

Ban wrote...

The buffer has the advantage that all 3 terminals of the input
transistors are at the same AC-potential, because the collectors
go to the output, thus a really wide bandwidth can be achieved.

That's an attractive aspect of this circuit. IRRC, I first saw
that cute trick in an NSC note in the late 60s.

I agree with Win, those are high-voltage parts good for +/-150V
not +/-10V. Siemens also makes some dual RF NPN and PNP with high
bandwidth, but I do not know if the supply voltage is sufficient.

Let's evaluate the scene a bit. 10V into 100 ohms is 100mA, rising
to 250mA and 3.1W if 12.5V via 50-ohms into an output short. The
transistor output-stage maximum power dissipation occurs for say
7.5V out (at the 50-ohm resistor input) at 150mA into low Z load,
for 1.1W at DC. Two sot-223 parts as shown may be fine, but three
would be a bit safer.

It'd be more interesting if the amplifier were spec'd for +/-10V
matched-impedance output into a 50-ohm load. We'd set it up for
a gain of 2x, with +/-20V output and at least +/-25V supplies.

.. ---+---+---------------+----+----+----+----- +25
.. | | 10mA | | | |
.. (I) (I) | | | |
.. 20mA | | _ |/ |/ |/ |/
.. | +---------U---|----|-----|----| fzt2222a
.. | | bead |> |> |> |>
.. fzt2222a | | | | | |
.. ,---)---)----, | | | |
.. |/ | \| | .-. .-. .-. .-. 4.7
.. +--| | |--+ | | | | | | | |
.. | |> | <| | | | | | | | | |
.. | '-+-)---' | '-' '-' '-' '-'
.. 249 | | | | 499 | | | | 50, 6W
.. o-/\/\--+ | | +--+-/\/\--+----+----+----+--/\/\---o
.. | | | | | | | | |
.. | ,-)-+---, | .-. 499 .-. .-. .-. .-. 4.7
.. | |< | >| | | | | | | | | | | |
.. +--| | |--+ | | | | | | | | | |
.. |\ | /| | '-' '-' '-' '-' '-'
.. '-)-----)----' | | | | |
.. fzt2907a | | gnd | | | |
.. | | _ |< |< |< |<
.. | +---------U---|----|----|----| fzt2907a
.. 20mA | | bead |\ |\ |\ |\
.. (I) (I) | | | |
.. | | 10mA | | | |
.. ---+-----+---------------+----+----+----+---- -25

The 60V Vcbo rating of the fzt2222a and fzt2907a would suffice.
One could run the input-pair current sources at 20mA and the
collector sources at 10mA, to insure enough available current
to drive the output emitter-follower transistors at full load.
The 20mA current sources could be made from two BJTs, so I get
a total transistor count of 22. The PCB would run rather hot.

Hmm, what does spice tell us about this circuit's performance?
You can use 2n2222 and 2n2907 models, they're the same dies.

 

[Chris Jones]

I trying to build a signal generator with 20 Vpp open circuit , 10Vpp on
50 Ohm output and frequency range up to 10MHz , so I need at least 30MHz
bandwith ( don't blame me if you think is a bit low !! )

I have found almost suitable parts in Analog Device AD815 or Texas
BUF634 plus high speep Op Amp.

Now I am looking for alternative implementation of output stage

An discrete or Op Amp + discrete output stage solution will be very
good if they have a small component count and if the components are easy
to find.


thanks in advance

I was interested in that kind of thing a few years ago because I had been
using a Wavetek 142, and I wanted to design something with similar specs,
using a MAX038 to generate the waveforms. The Wavetek used all discrete
transistors in the output section AFAIR.

What I came up with in the end was an Elantec EL2009 1Amp 90MHz gain=1
buffer inside the feedback loop of a EL2030 amplifier run with a fairly low
gain of about 3 to get good bandwidth. Then I had a second EL2030 before
that to give a bit more gain. As well as all that, I had an OP-07 to null
out the DC offsets of the whole arrangement. The problem is that now I
can't get the EL2009 any more.

So my advice is not to rely on the continued availability of fast, high
current buffer chips. Either use discrete transistors or buy more than
you'll ever need of the chip.

If you come up with a good solution I would like to see it please.

Chris

 

[Harry Dellamano]

It'd be more interesting if the amplifier were spec'd for +/-10V
matched-impedance output into a 50-ohm load. We'd set it up for
a gain of 2x, with +/-20V output and at least +/-25V supplies.

. ---+---+---------------+----+----+----+----- +25
. | | 10mA | | | |
. (I) (I) | | | |
. 20mA | | _ |/ |/ |/ |/
. | +---------U---|----|-----|----| fzt2222a
. | | bead |> |> |> |>
. fzt2222a | | | | | |
. ,---)---)----, | | | |
. |/ | \| | .-. .-. .-. .-. 4.7
. +--| | |--+ | | | | | | | |
. | |> | <| | | | | | | | | |
. | '-+-)---' | '-' '-' '-' '-'
. 249 | | | | 499 | | | | 50, 6W
. o-/\/\--+ | | +--+-/\/\--+----+----+----+--/\/\---o
. | | | | | | | | |
. | ,-)-+---, | .-. 499 .-. .-. .-. .-. 4.7
. | |< | >| | | | | | | | | | | |
. +--| | |--+ | | | | | | | | | |
. |\ | /| | '-' '-' '-' '-' '-'
. '-)-----)----' | | | | |
. fzt2907a | | gnd | | | |
. | | _ |< |< |< |<
. | +---------U---|----|----|----| fzt2907a
. 20mA | | bead |\ |\ |\ |\
. (I) (I) | | | |
. | | 10mA | | | |
. ---+-----+---------------+----+----+----+---- -25

The 60V Vcbo rating of the fzt2222a and fzt2907a would suffice.
One could run the input-pair current sources at 20mA and the
collector sources at 10mA, to insure enough available current
to drive the output emitter-follower transistors at full load.
The 20mA current sources could be made from two BJTs, so I get
a total transistor count of 22. The PCB would run rather hot.

Hmm, what does spice tell us about this circuit's performance?
You can use 2n2222 and 2n2907 models, they're the same dies.

Very nice Ban and Win. I count 20 transistors with the current sources
added. What have I missed? Please don't answer, "everything since the third
grade".
Regards,
Harry

 

[John Larkin]

I was interested in that kind of thing a few years ago because I had been
using a Wavetek 142, and I wanted to design something with similar specs,
using a MAX038 to generate the waveforms. The Wavetek used all discrete
transistors in the output section AFAIR.

What I came up with in the end was an Elantec EL2009 1Amp 90MHz gain=1
buffer inside the feedback loop of a EL2030 amplifier run with a fairly low
gain of about 3 to get good bandwidth. Then I had a second EL2030 before
that to give a bit more gain. As well as all that, I had an OP-07 to null
out the DC offsets of the whole arrangement. The problem is that now I
can't get the EL2009 any more.

So my advice is not to rely on the continued availability of fast, high
current buffer chips. Either use discrete transistors or buy more than
you'll ever need of the chip.

If you come up with a good solution I would like to see it please.

Chris

Just parallel 2 to 4 fast opamps.

John

 

[john jardine]

Ban wrote... [clip interesting bits]
It'd be more interesting if the amplifier were spec'd for +/-10V
matched-impedance output into a 50-ohm load. We'd set it up for
a gain of 2x, with +/-20V output and at least +/-25V supplies.

. ---+---+---------------+----+----+----+----- +25
. | | 10mA | | | |
. (I) (I) | | | |
. 20mA | | _ |/ |/ |/ |/
. | +---------U---|----|-----|----| fzt2222a
. | | bead |> |> |> |>
. fzt2222a | | | | | |
. ,---)---)----, | | | |
. |/ | \| | .-. .-. .-. .-. 4.7
. +--| | |--+ | | | | | | | |
. | |> | <| | | | | | | | | |
. | '-+-)---' | '-' '-' '-' '-'
. 249 | | | | 499 | | | | 50, 6W
. o-/\/\--+ | | +--+-/\/\--+----+----+----+--/\/\---o
. | | | | | | | | |
. | ,-)-+---, | .-. 499 .-. .-. .-. .-. 4.7
. | |< | >| | | | | | | | | | | |
. +--| | |--+ | | | | | | | | | |
. |\ | /| | '-' '-' '-' '-' '-'
. '-)-----)----' | | | | |
. fzt2907a | | gnd | | | |
. | | _ |< |< |< |<
. | +---------U---|----|----|----| fzt2907a
. 20mA | | bead |\ |\ |\ |\
. (I) (I) | | | |
. | | 10mA | | | |
. ---+-----+---------------+----+----+----+---- -25

The 60V Vcbo rating of the fzt2222a and fzt2907a would suffice.
One could run the input-pair current sources at 20mA and the
collector sources at 10mA, to insure enough available current
to drive the output emitter-follower transistors at full load.
The 20mA current sources could be made from two BJTs, so I get
a total transistor count of 22. The PCB would run rather hot.

Hmm, what does spice tell us about this circuit's performance?
You can use 2n2222 and 2n2907 models, they're the same dies.

Interesting arrangement. I remember Ban's original posting and look to use
it.
For 50ohm, 20Vpk, I started off with 30ma tails using paired BJTs. Output
drive of 15ma via folded cascodes.
For simplicity, the input tail currents ended up as a couple of power
resistors and caps to filter supply noise.
Had problems with output stage BJT heat distribution using low value emitter
resistors. Went upto 22 ohms, used 6 output pairs and a +/-30 volts supply.
Found additional cap' loading was limiting slew rate (feedback network),
cascode current already high, so added extra base driver pairs for the
o/p stage.

Fortunately, spice sim proved near to reality and allowed essential feedback
shaping (x18 gain, BJTs all BC546, BC556). A whole rat pack of
physical/electrical stability factors started biting at about 20-30MHz and
without the spice I think I'd have been buggered!.
regards
john

 

[Winfield Hill]

Harry Dellamano wrote...

Winfield Hill wrote ...

Very nice Ban and Win. I count 20 transistors with the
current sources added. What have I missed?

Use a better current-source, e.g., Wilson mirror, etc.

 

[Winfield Hill]

John Larkin wrote...

Just parallel 2 to 4 fast opamps.

At low frequencies this can be nicely done using the old ballast-
resistor current-sharing trick,

.. -- feedback network ---,
.. normal opamp |
.. ------| >--+--/\/--+-+----
.. _______| |
.. | add-on opamps |
.. | as followers |
.. +--| >-----/\/--+
.. | |
.. +--| >-----/\/--+
.. | |
.. '--| >-----/\/--'

But at high frequencies slew rate becomes an important issue.
For example, 20Vpp at 10MHz requires a better than 630V/us slew
rate, while delivering a full load. That's a rarefied territory
for +/-15V opamps. Generally opamps slewing that fast require a
large error voltage before their slew rate approaches that speed.

One has to be sure each opamp has the same high error voltage so
it slews in step with its neighbor. This could mean each opamp
to be shared needs its own independent feedback network, plus the
output ballast resistors shown above. Or some other trick would
be needed yielding identical circuitry for all the output opamps.

.. -- feedback network -------,
.. |
.. gain followers |
.. ----| >--+--| >-----/\/--+-+----
.. | |
.. +--| >-----/\/--+
.. | |
.. +--| >-----/\/--+
.. | |
.. '--| >-----/\/--'

BTW, one interesting +/-15V high-current (500mA) moderately-fast
(900V/us) opamp to consider is Analog Devices AD815 differential
ADSL driver, http://www.analog.com/en/prod/0,2877,AD815,00.html
This part can handle the task all by itself.

 

[John Larkin]

John Larkin wrote...

At low frequencies this can be nicely done using the old ballast-
resistor current-sharing trick,

. -- feedback network ---,
. normal opamp |
. ------| >--+--/\/--+-+----
. _______| |
. | add-on opamps |
. | as followers |
. +--| >-----/\/--+
. | |
. +--| >-----/\/--+
. | |
. '--| >-----/\/--'

But at high frequencies slew rate becomes an important issue.
For example, 20Vpp at 10MHz requires a better than 630V/us slew
rate, while delivering a full load. That's a rarefied territory
for +/-15V opamps. Generally opamps slewing that fast require a
large error voltage before their slew rate approaches that speed.

One has to be sure each opamp has the same high error voltage so
it slews in step with its neighbor. This could mean each opamp
to be shared needs its own independent feedback network, plus the
output ballast resistors shown above. Or some other trick would
be needed yielding identical circuitry for all the output opamps.

. -- feedback network -------,
. |
. gain followers |
. ----| >--+--| >-----/\/--+-+----
. | |
. +--| >-----/\/--+
. | |
. +--| >-----/\/--+
. | |
. '--| >-----/\/--'

BTW, one interesting +/-15V high-current (500mA) moderately-fast
(900V/us) opamp to consider is Analog Devices AD815 differential
ADSL driver, http://www.analog.com/en/prod/0,2877,AD815,00.html
This part can handle the task all by itself.

Maybe cast an eyeball at THS3062. +-15 supplies, 7000 v/usec, dual,
300 mhz, isolated powerpad. I like to run one section as a gain stage,
then three as pure followers (no overall fb) each with 150 ohms to the
output node. This is TI's fast 30-volt process, one of the few around.
They're rated at 140 mA or something, but really get droopy (and hot!)
if you push them that hard, hence the paralleled sections.

John

 

[Winfield Hill]

John Larkin wrote...

Maybe cast an eyeball at THS3062. +-15 supplies, 7000 v/usec, dual,
300 MHz, isolated powerpad. I like to run one section as a gain stage,
then three as pure followers (no overall fb) each with 150 ohms to the
output node. This is TI's fast 30-volt process, one of the few around.
They're rated at 140 mA or something, but really get droopy (and hot!)
if you push them that hard, hence the paralleled sections.

Yes, very nice. John, can you tell us your recommendations for
effectively using TI's PowerPad thermal path in prototype assembly?