Switchable inverting/non-inverting opamp circuit?

[Richard Rasker]

Hello,

I recall having seen a rather simple circuit with 2 opamps (IIRC) which
could switch from inverting to non-inverting operation by means of
switching a single transistor or FET on and off -- but I can't find it
anywhere anymore.

For a particular type of ramp generator, I need to switch electronically
between two voltages, mirrored around a reference voltage of 1.25V -- e.g.
1.0V and 1.5V, or 0.7V and 1.8V etcetera.

Sure, I can build a simple opamp inverter with 1.25V at its + input, and use
a 4053 two-channel multiplexer gate to switch between the input and output
voltages, but that seems like a bit of waste in terms of devices and PCB
space.

Can anyone remember how this was done? Thanks already for any suggestions.

Richard Rasker

 

[Richard Rasker]

ftp://jjlarkin.lmi.net/Gain_PM1.JPG

Yup, that was the one I meant -- so simple it's almost alarming that I
couldn't remember. I must be getting old :-/
I can replace the switch with a small-signal MOSFET, and have the exact
desired solution -- a few thents of an ohm Rdson are no problem with R=10K.

or

ftp://jjlarkin.lmi.net/Gain_PM1_b.JPG


But the circuit you suggest, the inverter and the downstream mux, are
about equivalent. This does have a lower output impedance in both
states.

Indeed it does -- but I think I'll just go with the first solution.

Anyway, thanks so much, now I can get on with the really tricky bits

Richard Rasker

 

[Richard Rasker]

John Larkin wrote:

[snip switchable inverter/non-inverter]

Aren't we all?

That circuit makes a nice synchronous detector.

It sure does.

....

Anything fun?

It's a triangle wave sweep generator, with the lowest and highest voltage
levels presettable by means of (multiturn) potmeters, and a reasonably
fixed frequency of 1 Hz. IOW: it doesn't speed up or down with a smaller
respectively wider sweep range, which is essential for reasons that I'm not
allowed to discuss. Here's the schematic of most of it:

http://www.linetec.nl/electronics/sweep-page1.ps

I think this'll work (several parts are already built in a breadboard setup
and tested), although I still have to double-check the polarity of the
window detector / flip flop part at the center, controlling the MOSFET in
your [non-]inverter.

Richard Rasker

 

[Richard Rasker]

John Larkin wrote:

[snip switchable inverter/non-inverter]

Aren't we all?

That circuit makes a nice synchronous detector.

It sure does.

...

Anything fun?

It's a triangle wave sweep generator, with the lowest and highest voltage
levels presettable by means of (multiturn) potmeters, and a reasonably
fixed frequency of 1 Hz. IOW: it doesn't speed up or down with a smaller
respectively wider sweep range, which is essential for reasons that I'm
not allowed to discuss. Here's the schematic of most of it:

http://www.linetec.nl/electronics/sweep-page1.ps

I think this'll work (several parts are already built in a breadboard
setup and tested), although I still have to double-check the polarity of
the window detector / flip flop part at the center, controlling the MOSFET
in your [non-]inverter.

And right after posting this, I spotted a dumb error: in 'Set' mode, the
MOSFET should be permanently switched on. Ah well, it's getting late --
I'll fix that tomorrow ...

Richard Rasker

 

[John Devereux]

John Larkin wrote:

[snip switchable inverter/non-inverter]

Can anyone remember how this was done? Thanks already for any
suggestions.

Richard Rasker

ftp://jjlarkin.lmi.net/Gain_PM1.JPG

Yup, that was the one I meant -- so simple it's almost alarming that I
couldn't remember. I must be getting old :-/

Aren't we all?

That circuit makes a nice synchronous detector.

It sure does.

[snip]

I put it in a hybrid for Hughes (Tucson) in 1971.

There's also a more accurate way. I'd post it but Larkin would claim it as
his own ;-) Interested parties inquire privately.

FFS Jim why do you bother posting here anymore?

 

[John Devereux]

John Larkin wrote:

[snip switchable inverter/non-inverter]

Can anyone remember how this was done? Thanks already for any
suggestions.

Richard Rasker

ftp://jjlarkin.lmi.net/Gain_PM1.JPG

Yup, that was the one I meant -- so simple it's almost alarming that I
couldn't remember. I must be getting old :-/

Aren't we all?

That circuit makes a nice synchronous detector.

It sure does.

[snip]

I put it in a hybrid for Hughes (Tucson) in 1971.

I put it into the Boresight Alignment Kit for the C5A around 1968 or
so.

There's also a more accurate way. I'd post it but Larkin would claim it as
his own ;-) Interested parties inquire privately.

Hell, he never posts circuits any more; he just brags about past
glories. He's using me as an excuse for not having ideas. Senile old
fool.

What's a more accurate way to flip a DC voltage than using an opamp
and two equal-value resistors?

Switched capacitor? See e.g. fig 20 in

<http://cds.linear.com/docs/Application Note/an03f.pdf>

1ppm accuracy with no precision parts!

Pretty cool.

 

[John Devereux]

John Larkin wrote:

[snip switchable inverter/non-inverter]

Can anyone remember how this was done? Thanks already for any
suggestions.

Richard Rasker

ftp://jjlarkin.lmi.net/Gain_PM1.JPG

Yup, that was the one I meant -- so simple it's almost alarming that I
couldn't remember. I must be getting old :-/

Aren't we all?

That circuit makes a nice synchronous detector.

It sure does.

[snip]

I put it in a hybrid for Hughes (Tucson) in 1971.

There's also a more accurate way. I'd post it but Larkin would claim it as
his own ;-) Interested parties inquire privately.

FFS Jim why do you bother posting here anymore?

I presume your E-mail address is accurate? I'll send you a schematic.
Yes

If you were paying attention, I do respond to _real_ requests for
information. But there's VERY few such requests anymore.

Larkin can't stop "cluck clucking", so I can't help but slam the little SOB.
When he shuts up, so will I.

When he shuts up? As far as I can see he gave the OP exactly what he was
after (the simple optional inverter circuit). Which is more than you
did.

I don't doubt there are other solutions, I posted one myself that was
"more accurate" at DC (depending on assumptions). But the OP was asking
for a specific circuit configuration he couldn't remember, it's the one
that everyone learns and JL at least took the trouble to sketch it and
post it.

What have YOU designed lately? That worked as requested ? I've designed
nearly a whole SOC in the past two weeks.

Nothing very exciting I'm afraid. I think the last actual analog
electronics included the 4-20mA driver circuit that I asked for ideas
about a while ago, here.

I do have to do a lot of other stuff, programming, PCB layout, admin...

 

[John Devereux]

On Tue, 26 Jul 2011 18:26:02 -0400, Jim Thompson

John Larkin wrote:

[snip switchable inverter/non-inverter]

Can anyone remember how this was done? Thanks already for any
suggestions.

Richard Rasker

ftp://jjlarkin.lmi.net/Gain_PM1.JPG

Yup, that was the one I meant -- so simple it's almost alarming that I
couldn't remember. I must be getting old :-/

Aren't we all?

That circuit makes a nice synchronous detector.

It sure does.

[snip]

I put it in a hybrid for Hughes (Tucson) in 1971.

I put it into the Boresight Alignment Kit for the C5A around 1968 or
so.


There's also a more accurate way. I'd post it but Larkin would claim it as
his own ;-) Interested parties inquire privately.

Hell, he never posts circuits any more; he just brags about past
glories. He's using me as an excuse for not having ideas. Senile old
fool.

What's a more accurate way to flip a DC voltage than using an opamp
and two equal-value resistors?

Switched capacitor? See e.g. fig 20 in

<http://cds.linear.com/docs/Application Note/an03f.pdf>

1ppm accuracy with no precision parts!

Pretty cool.

Flying-capacitor circuits are cool. There are a couple of gotchas,
mostly related to charge injection and dielectric absorption.

Not in this circuit though I think.

But no resistor ratios to worry about.

That's the key, 1ppm resistors are kind of pricy

 

[Richard Rasker]

It's a triangle wave sweep generator, with the lowest and highest voltage
levels presettable by means of (multiturn) potmeters, and a reasonably
fixed frequency of 1 Hz. IOW: it doesn't speed up or down with a smaller
respectively wider sweep range, which is essential for reasons that I'm
not allowed to discuss. Here's the schematic of most of it:

http://www.linetec.nl/electronics/sweep-page1.ps

I think this'll work (several parts are already built in a breadboard
setup and tested), although I still have to double-check the polarity of
the window detector / flip flop part at the center, controlling the
MOSFET in your [non-]inverter.

You would be better off using a fixed amplitude and frequency triangle
waveform generator followed by an adjustable gain amplifier/buffer to
set the output amplitude, instead of running those adjustable inputs
all over the place...

That would admittedly be much easier, but one of the prerequisites in this
case is that the lowest and highest output values can be set (semi-)
independently and accurately. I found no way of doing this using adjustable
gain and offset.


Richard Rasker

 

[Fred Bartoli]

Richard Rasker a écrit :

It's a triangle wave sweep generator, with the lowest and highest voltage
levels presettable by means of (multiturn) potmeters, and a reasonably
fixed frequency of 1 Hz. IOW: it doesn't speed up or down with a smaller
respectively wider sweep range, which is essential for reasons that I'm
not allowed to discuss. Here's the schematic of most of it:

http://www.linetec.nl/electronics/sweep-page1.ps

I think this'll work (several parts are already built in a breadboard
setup and tested), although I still have to double-check the polarity of
the window detector / flip flop part at the center, controlling the
MOSFET in your [non-]inverter.

You would be better off using a fixed amplitude and frequency triangle
waveform generator followed by an adjustable gain amplifier/buffer to
set the output amplitude, instead of running those adjustable inputs
all over the place...

That would admittedly be much easier, but one of the prerequisites in this
case is that the lowest and highest output values can be set (semi-)
independently and accurately. I found no way of doing this using adjustable
gain and offset.

How about having your switchover points fixed as you want. Then just
derive the integrated current values from the switch points deltaV.

Trim to get 1Hz
|
G=1 inst amp | ||
| .----||-.
HC4053 V | || |
|\ +--------o ___ | |\ |
Vlow >-------|-\ | .----. \o-|___|-+--|-\ |
| >-+-| -1 |-o | >-+
Vhigh >-------|+/ '----' ^ GND-|+/ |
|/ | |/ |
| |
| /| |
| .---. /+|------< Vlow
| | S|--< | |
| | | \-|--+
'--o| R|-. \| |
'---' | |
| /| |
| /+|--'
'< |
\-|------< Vhigh
\|

 

[Richard Rasker]

Richard Rasker a écrit :

It's a triangle wave sweep generator, with the lowest and highest
voltage levels presettable by means of (multiturn) potmeters, and a
reasonably fixed frequency of 1 Hz. IOW: it doesn't speed up or down
with a smaller respectively wider sweep range, which is essential for
reasons that I'm not allowed to discuss. Here's the schematic of most
of it:

http://www.linetec.nl/electronics/sweep-page1.ps

I think this'll work (several parts are already built in a breadboard
setup and tested), although I still have to double-check the polarity
of the window detector / flip flop part at the center, controlling the
MOSFET in your [non-]inverter.
You would be better off using a fixed amplitude and frequency triangle
waveform generator followed by an adjustable gain amplifier/buffer to
set the output amplitude, instead of running those adjustable inputs
all over the place...

That would admittedly be much easier, but one of the prerequisites in
this case is that the lowest and highest output values can be set (semi-)
independently and accurately. I found no way of doing this using
adjustable gain and offset.

How about having your switchover points fixed as you want. Then just
derive the integrated current values from the switch points deltaV.

Trim to get 1Hz
|
G=1 inst amp | ||
| .----||-.
HC4053 V | || |
|\ +--------o ___ | |\ |
Vlow >-------|-\ | .----. \o-|___|-+--|-\ |
| >-+-| -1 |-o | >-+
Vhigh >-------|+/ '----' ^ GND-|+/ |
|/ | |/ |
| |
| /| |
| .---. /+|------< Vlow
| | S|--< | |
| | | \-|--+
'--o| R|-. \| |
'---' | |
| /| |
| /+|--'
'< |
\-|------< Vhigh
\|

Fred,
Not to put too fine a point on it, but this is pretty much what I came up
with in the first place (refer to
http://www.linetec.nl/electronics/sweep-page1.ps):
- U3 p8/9/10 provides the difference between Vlow and Vhigh
- U3 p1/2/3 is the switchable inverter/non-inverter, with a MOSFET
instead of a 4053
- U3 p12/13/14 is the integrator
- U2 p1/2/3 and p12/13/14 are the comparators
- U2 p5/6/7 is configured as flip-flop

But thanks for your effort anyway -- you've come up with some real gems in
the past, among which a very elegant log converter circuit for a mass air
flow sensor, which is still in use

Richard Rasker

 

[Fred Bartoli]

Richard Rasker a écrit :

Richard Rasker a écrit :

Fred Bloggs wrote:

It's a triangle wave sweep generator, with the lowest and highest
voltage levels presettable by means of (multiturn) potmeters, and a
reasonably fixed frequency of 1 Hz. IOW: it doesn't speed up or down
with a smaller respectively wider sweep range, which is essential for
reasons that I'm not allowed to discuss. Here's the schematic of most
of it:

http://www.linetec.nl/electronics/sweep-page1.ps

I think this'll work (several parts are already built in a breadboard
setup and tested), although I still have to double-check the polarity
of the window detector / flip flop part at the center, controlling the
MOSFET in your [non-]inverter.
You would be better off using a fixed amplitude and frequency triangle
waveform generator followed by an adjustable gain amplifier/buffer to
set the output amplitude, instead of running those adjustable inputs
all over the place...
That would admittedly be much easier, but one of the prerequisites in
this case is that the lowest and highest output values can be set (semi-)
independently and accurately. I found no way of doing this using
adjustable gain and offset.

How about having your switchover points fixed as you want. Then just
derive the integrated current values from the switch points deltaV.

Trim to get 1Hz
|
G=1 inst amp | ||
| .----||-.
HC4053 V | || |
|\ +--------o ___ | |\ |
Vlow >-------|-\ | .----. \o-|___|-+--|-\ |
| >-+-| -1 |-o | >-+
Vhigh >-------|+/ '----' ^ GND-|+/ |
|/ | |/ |
| |
| /| |
| .---. /+|------< Vlow
| | S|--< | |
| | | \-|--+
'--o| R|-. \| |
'---' | |
| /| |
| /+|--'
'< |
\-|------< Vhigh
\|

Fred,
Not to put too fine a point on it, but this is pretty much what I came up
with in the first place (refer to
http://www.linetec.nl/electronics/sweep-page1.ps):
- U3 p8/9/10 provides the difference between Vlow and Vhigh
- U3 p1/2/3 is the switchable inverter/non-inverter, with a MOSFET
instead of a 4053
- U3 p12/13/14 is the integrator
- U2 p1/2/3 and p12/13/14 are the comparators
- U2 p5/6/7 is configured as flip-flop

Ahem, OK, I missed it. Pretty much the same indeed.

But thanks for your effort anyway -- you've come up with some real gems in
the past, among which a very elegant log converter circuit for a mass air
flow sensor, which is still in use

Glad it helped

 

[Kaz Kylheku]

It seems like the circuit on page 2 of this PDF would do the trick
(basically the same as the one shown in the sweep-page1.ps document), but
A_abs is usually less than 100 mV and the error introduced by the V_CE≠0
adds an offset onto the output.

Hi sclarke. Forget the bipolar transistor. Although some have a very low
VCE(sat), especially when the collector current is small, it could still
be significant w.r.t. 0.1V.

Consider a MOSFET, or an IC providing analog switches.

With some analog switching/multiplexing you could also implement a
solution whereby you split the signal between a buffer and inverter, and
then multiplex on the output side.