I'm studying Bob Parker's ESR Meter Mk2 and got caught in a part of the circuit that he calls pulse amplifier, which schematic I copy below:
Figure 1: Bob Parker's ESR Pulse Amplifier
As far as I can tell, the meter submits the capacitor under test to short pulses of constant current that will create a voltage across the cap terminals that is proportional to its Equivalent Series Resistance (ESR).
That voltage (of about a few tens of mV) is then input into the so called pulse amplifier of gain 20. See the explanation copied from the text.
The current pulses developed across the test capacitor are fed via C7 and R12 to a fast non-inverting pulse amplifier based on transistors Q7 and Q8. These two transistors are wired as common-emitter stages, with feedback applied via R17 to give an overall gain of about 20, depending on the setting of VR2.
Well, I'm not satisfied. The explanation was a bit too economical for me. So, my question is:
- How does this pulse amplifier work?
I'm a bit familiar with using OpAmps to amplify signals, but I have no idea how this circuit above does it's tricks.
Simple Google searches using "pulse amplifier" as keywords have not been helpful as I get directed to very different kinds of circuits.
To be able to see the signals on my oscilloscope, and maybe clarify things a bit, I went ahead and built the circuit in a breadboard. I fed it with a 10kHz square wave from a signal generator (yellow trace) and the result was the following scope shots. The green trace is tied to the PULSE SIG MCU input as shown in the schematic.
Well, that was even more puzzling as the input pulse doesn't seem to have been amplified 20 times as I was let to believe it should have. Also, I seem to be getting a short spike when the input signal goes down. Is this output expected from the pulse amplifier or is my circuit messed up beyond salvation?
I did scrap the breadboard circuit and built it again, fed it with an 8us pulse at 2kHz, just like the circuit expects it (like in Spehro's simulation), and got much better results. See scope shots below.
Apart from any circuit errors, I did seem to be overdriving the amplifier both in input voltage and in duty cycle. Now it seems to be amplifying the pulses alright.