I solved the motorboating, but still feel like I'm not getting enough gain.
First, regarding the motorboating: Previously the AGC amplifier was built on a solderless breadboard, and I suspected my motorboating problems may have been related to ground loops caused by small differences in ground potentials of the transistors, differences caused by the imperfect nature of the mechanical contacts on a solderless breadboard.
I rebuilt the amplifier using "ugly construction" techniques over a copper ground plane. Then, ahead of the AGC amp, I added a regenerative detector, an emitter-follower AF buffer, and a single common-emitter AF amp. Specifically, I added transistors Q2, Q3, and Q4 from my Vackar-style regenerative receiver, then connected the collector output of Q4 to the input of the C. Hall AGC amp.
The circuit diagram looks as follows.
The physical construction looks as follows. The top half of the breadboard is the Vackar-style regenerative detector (Q2); the bottom half of the breadboard is the AF part of the circuit: buffer, preamp, and AGC amp. The ugly construction techniques used for the AF part of the circuit would probably be disastrous with an RF circuit, but for a low-frequency AF amplifier I can take some liberties with leaving component leads long and not worrying too much about stray couplings between input and output.
This physical construction was, unlike the solderless breadboard version, mostly stable in terms of not having unwanted AF oscillations. However, if the input to the AF buffer (C2) was disconnected, then the AF chain had a tendency for a hissing or sputtering oscillation. But with the AF buffer connected to the regenerative detector, all was well.
The AF amp provided enough gain to listen to the regenerative receiver at the output of the AGC amp. (Input for the regenerative detector was a loop antenna coupled into the tank inductor by a link winding.)
However, I wasn't getting sufficient AGC action yet. The AGC action can be monitored by observing the base voltage at Q8 of the AGC amp. It should shoot all the way up to Vcc (1.2 V) on strong signals, but right now it's not doing that.
The problem is that the AF input to the AGC amp has to be large enough to trigger the AGC action. The AF output from my regenerative detector Q2 will be very small, and the Q3 buffer offers no voltage gain, so only one transistor, Q4, provides the gain for the AF signal that will be input into the AGC amp.
So the solution is to add more pre-amplification after Q4 and before the AGC amp input at C2, to boost the signal levels coming into the AGC amp to a level such that even moderately strong signals already start to trigger the AGC action.
The complexity of the AF part of the circuit is already somewhat high (8 transistors), and with additional pre-amplification that will rise to 9 or possibly even 10 transistors. That's a rather complex circuit, but once the AF amp is built, it can be re-used with any simple one-transistor regenerative detector. So the AF amp complexity can be hidden and just considered to be a black-box functionality, allowing future design effort to focus on the regenerative detector only.
And speaking of design efforts focusing on the regenerative detector, I've been doing some investigation of automatic regeneration control and have some promising circuit candidates working in the LTspice simulator. Once I perfect the AGC AF amp, I will work on verifying my automatic regeneration control ideas in hardware.
I added another common-emitter preamp. Furthermore, I added a 100 microamp moving-coil meter between between the Q1 base and the Q9 emitter to monitor the AGC current. However, I'm still not getting the AGC action I expect.
In my LTspice simulations the Q1 base current should be around 6 uA for an input signal of 10 mV, jumping up to 40 uA for an input signal of 100 mV, and leveling out at around 50 uA for any higher signal level.
In practice I only observed mostly no base current, but if I ran a local signal generator near the regen (injecting a large, powerful signal), I could barely get around 10 uA to flow for a moment. This indicates to me that my input signal levels, even with two common-emitter stages of amplification, are still too low.
However, I'm starting to be troubled by noise in the amplifier. Broadly speaking, the problem can likely be isolated to one of two locations: either the preamp stages, or the AF AGC amp itself. (This assumes the noise is not due to some interaction between the two stages.) I suspect the preamp is noisy.
My plan of attack is as follows.
- Add another common-emitter preamp, for 3 stages of common-emitter amplification.
- Connect headphones to the output of the preamp (possibly using an additional power amplifier to boost the signal level) and confirm that no abnormal noise is present. If noise is present, solve this first by rebuilding the preamp with proper attention to neat layout and power supply decoupling.
- Measure AF signal amplitude at the output of the 3-state common-emitter preamp and confirm that it is between 10 mV and 100 mV for typical signals.
- Connect preamp to the AGC amp and confirm no abnormal noise is present.
- Confirm that between 10 uA and 50 uA of base current is flowing into Q1 on strong signals.
Update 2, 2015-12-06
I'm pretty sure the "noise" in the AF chain is spurious oscillation. The AF chain can even go into fringe howl at some settings of regeneration. This is somewhat reminiscent of the problems I had with the earlier circuit version on the solderless breadboard: the AF AGC amp worked fine by itself, but adding preamp stages caused motorboating. I thought my new soldered construction had solved the motorboating, but it now seems it's not completely solved after all.
Again, I will solve this step by step. First, make a 3-stage common-emitter preamp, make that stable, then try to connect the AGC AF amp.
Update 3, 2015-12-07
When solving a complex problem, it's good to step back periodically and ask yourself if what you're doing actually makes sense. Doing some more experiments and simulations, I think I will need 10-11 transistors (1 buffer, 4 preamp stages, 6-transistor AGC amp) to get the AGC amp working as I expect, and even then the AGC dynamic range will be limited to around 30 dB before clipping distortion begins. In other words, I need 11 transistors for a limited AGC action. Since the original intent of this investigation was to to protect my ears when tuning across loud signals, a much simpler solution would simply be to use a germanium diode limiter across the AF amp output. That could probably be implemented with 6 transistors (1 buffer and 5 preamp stages to bring the AF output up to the ~300 mV level required for germanium diodes to clip).
I probably will continue to develop this AGC amp an an educational exercise, but in practice it may be that a diode limiter is the simpler, more effective solution.
Update 4, 2015-12-08
I took some more measurements. I took speaker-level output from the headphone jack of a portable transistor radio, and fed that into the AGC amp without any preamplification (directly into C2).
Setting the portable radio volume to maximum, AGC base current into Q1 was 40 microamps, with heavy distortion audible at the output of the AGC amp. Therefore the measured maximum AGC base current is 40 microamps, compared with a simulated maximum of 50 microamps.
The actual AF voltage level at the input of the AGC amp could not be measured due to unreliable readings from my multimeter, but I believe it is around 1.2 volts.
Reducing the volume of the portable radio such that AGC current was 30 microamps or 20 microamps still yielded noticeable distortion, especially on music.
Reducing the volume of the portable radio such that AGC current was 10 microamps or lower yielded mostly clean-sounding audio, even on music.
During speech, the AGC current correctly fluctuated down during pauses and back up again during words.
Conclusions for now:
- Distortion is clearly present over half of the AGC range (Q1 base current between 20 and 40 microamps). The distortion-free dynamic range is therefore rather small. I should fix my multimeter (which probably has a weak battery) and measure the distortion-free AF voltage range, but my feeling is that the dynamic range between weak and strong signals (that I want to hear on shortwave) will exceed the small distortion-free dynamic range I observed.
- The AGC amp requires a quite high headphone-level signal for effective AGC action.
- Given the distortion and the requirement for large amounts of pre-amplification, it is questionable whether investing more effort into this amplifier is warranted. In practice, even when using this AGC amp, it will be likely necessary to frequently and manually reduce signal levels coming into the amplifier to prevent distortion. Since frequent manual adjustment of signal levels is required, the low-dynamic-range AGC brings little operational benefit over a simple diode clipper.