Low Signal Amplification: OP-07 & LM324 Preamp

[Bob S]

Here is the SPICE model (see thumbnails) of the common-emitter version of the weird amp. The low frequency response is very sensitive to the capacitor bypass on the two emitters. With 4.7k emitter resistors (corresponding to about 1 mA collector current per transistor), the bypass had to be about 1000 uF to make the frequency response flat with minimal phase shift.
For posting these thumbnails from SPICE, I do a Print Screen of each SPICE image, paste them into Power Point, and save them as jpg.
Bob S
[Edit] As before, this circuit requires a floating 9V battery.

 

[Bob S]

Here is a redo of the common-emitter amplifier circuit in the previous post, except that the 9-volt battery has been separated from the collector signals, and the power supply bypassed to ground, so now this circuit can be coupled to a ground-referenced dc supply. Performance is about the same.

 

[vk6kro]

Thanks Bob, great simulations.

The second one looks a little more friendly although I find it hard to get away from a ground-referenced way of thinking.
I'm just a little worried that this may be a simulator artefact.

Did you get an idea of the input impedance for each common emitter setup?

 

[Bob S]

Here are some numbers for the last circuit. I put a resistor in series with the input voltage source, and found the corresponding output amplitudes:

0 ohms, -23.0 dB (below 1 volt)
1k ohms, -25.3 dB
2k ohms, -26.3 dB
5k ohms, -29.9 dB
10 k ohms, -33.7 dB

So 5K-ohm series input impedance is about half amplitude (6 dB down).

In the dc operating point measurement, the positive and negative rails are +3.76 volts and -5.24 volts, meaning the input would have a dc offset if the circuit runs on ground-referenced power supplies, so an input offset balance pot might be needed.

[Edit] I put two center-grounded 4.5 volt power supplies in series to replace the 9-volt battery, and the offset voltage across a 5k series input resistor was 6.8 mV, so there is an input bias current of 1.4 microamps.
Bob S