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OpAmp TTL problem

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marty1

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I have this inverting Op Amp circuit. When I input a TTL signal which goes from 0 to +5 volts, I expect the output to be inverted and go from 0 t0 -9 volts.

What I see on the O'scope is a square wave going from +9 volts to -9 volts.

Why is this?
TTL Problem.png
TTL Problem.png TTL Problem.png
 
Do you know how to calculate the gain of your circuit? It appears to be more than ten times what you want.

Also by capacitive coupling the input you change the signal from 0 to +5V to +/- 2.5V (the capacitor blocks the average DC voltage of the signal, look at the output of C1), thus the output will also be plus and minus (but inverted), not zero to minus.

Why do you have such an elaborate filter on the 9V to the input?
 
Yes the gain is too high. But even with the gain at 47, 0v in = 0v out, +5v in -9v out.

The 470k could be changed to 20k for a gain of 2.
Then 0v in = 0v out, +5v in = -9 v out.
 
Yes the gain is too high. But even with the gain at 47, 0v in = 0v out, +5v in -9v out.

The 470k could be changed to 20k for a gain of 2.
Then 0v in = 0v out, +5v in = -9 v out.
Yes, I forgot that it's operating from 9V supplies so the output will just limit at that value.
 
The output of an LT1013 opamp with a +/-9V supply does not go near +9V.
 
All,
Thanks for your help!

The gain is high because this part of a larger circuit. A microphone is actually attached to the input. I was just testing the circuit with the TTL signal.

Audioguru is correct this opamp is not rail to rail. I was just being lazy by writing +/- 9 volts rather than several volts below the rail voltage.
 
A microphone is actually attached to the input. I was just testing the circuit with the TTL signal.
I was just being lazy by writing +/- 9 volts rather than several volts below the rail voltage.

One of the joys here on ETO is finding out what the REAL QUESTION should have been.
It usually takes a few more iterations than this. Well done.
[/SARCASM]

JimB
 
The 2.2k resistor for powering the Jfet inside an electret mic has a value too low so it loads down the signal level from the mic. Use 10k.
The opamp is inverting with an input resistance of only 10k which also loads down the signal level from the mic. Use a non-inverting circuit with an input resistance of 50k.

The high value of the 10uF input capacitor C1 is silly because it passes earthquake frequencies as low as 1.6Hz.
 
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