Re: Help! Capacitive coupled transistor setup

Terradestroyer@xxxxxxxxx wrote: 
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
.

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