Help! Capacitive coupled transistor setup

[[email protected]]

I'm having a lot of trouble with this circuit.

http://www.engr.mun.ca/~wakeham/transistor.gif

It is the reference circuit from Kodak for their CCD imaging sensors.
What I need is when a logic level of 5v is applied to the left just
before the 33pf caps I need to get 5 volt out, and when 0v is applied I
need to get -3.5 volt from the Reset_CCD line.

So I connected my supply so that R-High was 5V and R-Low was -3.5V. The
problem is that no matter what I try I can only get the -3.5V from the
output. I thought it was a simple push-pull setup but the voltages
applied to the transistors are opposite to what would be expected.

I have a feeling that this circuit might only work at high speeds,
hence the very low capacitance of the coupling cap, but I don't have an
oscillascope to test it out with.

Its driving what is almost an entirely capacitive load of about 65 pf
at about a 30-40Mhz square wave.

Should this circuit give me the 5V and -3.5V output measureable by a
simple multimeter when 0-5V TTL logive voltages are applied or do I
need an oscillascope and waveform generator to confirm if this is
working.

Any help would be greatly apperciated because I need to build a bunch
of these to drive a CCD sensor.

Thank you greatly
Keith Wakeham

 

[Fred Bloggs]

I'm having a lot of trouble with this circuit.

http://www.engr.mun.ca/~wakeham/transistor.gif

It is the reference circuit from Kodak for their CCD imaging sensors.
What I need is when a logic level of 5v is applied to the left just
before the 33pf caps I need to get 5 volt out, and when 0v is applied I
need to get -3.5 volt from the Reset_CCD line.

That circuit gives R_High out when TTL goes low, and R_Low out when TTL
goes high.

So I connected my supply so that R-High was 5V and R-Low was -3.5V. The
problem is that no matter what I try I can only get the -3.5V from the
output. I thought it was a simple push-pull setup but the voltages
applied to the transistors are opposite to what would be expected.

I have a feeling that this circuit might only work at high speeds,
hence the very low capacitance of the coupling cap, but I don't have an
oscillascope to test it out with.

Its driving what is almost an entirely capacitive load of about 65 pf
at about a 30-40Mhz square wave.

Should this circuit give me the 5V and -3.5V output measureable by a
simple multimeter when 0-5V TTL logive voltages are applied or do I
need an oscillascope and waveform generator to confirm if this is
working.

Any help would be greatly apperciated because I need to build a bunch
of these to drive a CCD sensor.

Thank you greatly
Keith Wakeham

The transistors are common-emitter which you should recall are polarity
inverters.

 

[Tim Wescott]

I'm having a lot of trouble with this circuit.

http://www.engr.mun.ca/~wakeham/transistor.gif

It is the reference circuit from Kodak for their CCD imaging sensors.
What I need is when a logic level of 5v is applied to the left just
before the 33pf caps I need to get 5 volt out, and when 0v is applied I
need to get -3.5 volt from the Reset_CCD line.

So I connected my supply so that R-High was 5V and R-Low was -3.5V. The
problem is that no matter what I try I can only get the -3.5V from the
output. I thought it was a simple push-pull setup but the voltages
applied to the transistors are opposite to what would be expected.

I have a feeling that this circuit might only work at high speeds,
hence the very low capacitance of the coupling cap, but I don't have an
oscillascope to test it out with.

Its driving what is almost an entirely capacitive load of about 65 pf
at about a 30-40Mhz square wave.

Should this circuit give me the 5V and -3.5V output measureable by a
simple multimeter when 0-5V TTL logive voltages are applied or do I
need an oscillascope and waveform generator to confirm if this is
working.

Any help would be greatly apperciated because I need to build a bunch
of these to drive a CCD sensor.

Thank you greatly
Keith Wakeham

You really need to look at the thing closely and think about how it works.

With no activity on the input lead both transistors will turn off and
R113 will pull the output to R_LOW. When the input lead pulses high Q13
will turn on, actively pulling the output low. When the input lead
pulses low Q14 will turn on, actively pulling the output high. Should
the input pulse rapidly up and down there is a good chance that at some
part of the cycle both transistors will be on, making things toasty and
pulling lots of current. Base currents on the transistors will be
limited only by the rise time and/or source impedance of the input signal.

 

[Peter Bennett]

I'm having a lot of trouble with this circuit.

http://www.engr.mun.ca/~wakeham/transistor.gif

Should this circuit give me the 5V and -3.5V output measureable by a
simple multimeter when 0-5V TTL logive voltages are applied or do I
need an oscillascope and waveform generator to confirm if this is
working.

No. The input is AC coupled by the two 33pF capacitors, so a DC input
will have no effect. You will have to drive it near the intended
operating frequency, and observe the output with an oscilliscope.

 

[ehsjr]

I'm having a lot of trouble with this circuit.

http://www.engr.mun.ca/~wakeham/transistor.gif

It is the reference circuit from Kodak for their CCD imaging sensors.
What I need is when a logic level of 5v is applied to the left just
before the 33pf caps I need to get 5 volt out, and when 0v is applied I
need to get -3.5 volt from the Reset_CCD line.

So I connected my supply so that R-High was 5V and R-Low was -3.5V. The
problem is that no matter what I try I can only get the -3.5V from the
output. I thought it was a simple push-pull setup but the voltages
applied to the transistors are opposite to what would be expected.

I have a feeling that this circuit might only work at high speeds,
hence the very low capacitance of the coupling cap, but I don't have an
oscillascope to test it out with.

Its driving what is almost an entirely capacitive load of about 65 pf
at about a 30-40Mhz square wave.

Should this circuit give me the 5V and -3.5V output measureable by a
simple multimeter when 0-5V TTL logive voltages are applied or do I
need an oscillascope and waveform generator to confirm if this is
working.

Any help would be greatly apperciated because I need to build a bunch
of these to drive a CCD sensor.

Thank you greatly
Keith Wakeham

It's back-asswards from what you want. To see the operation,
connect a 100 ohm resistor and LED in series (make two
resistor/LED assemblies), and connect one assembly in series
with the emitter of each transistor and its corresponding
circuit, observing proper LED polarity. Next connect a 3.3K
resistor to the base of one of the transistors. Touch the other
end of the resistor to V high and observe the LEDS, then touch
it to ground and observe. Now move it to the base of the other
transistor and repeat. That will give you a visual indication
of the polarity that will cause each transistor to conduct,
without needing to worry about pulses. It will also verify that
the transistor circuit is wired properly according to the
schematic, and that the transistor is working. If the test does
not result in seeing each LED light, when the base of its
corresponding transistor is connected by the resistor to either
High or ground, it means the circuit is wired wrong or there
is a bad part.

Ed

 

[[email protected]]

From what I've been testing now and understanding the circuit works. I
apperciate all the help and explanations from everyone. Its a reference
design for a reason, but I'd rather not blindly trust it.

Thanks again
Keith