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Analog Electronics - Noisy RegenThe Noisy Regen circuit as described by Alan Yates is a simple and easy to construct regenerative radio receiver. In addition it is a very repeatable circuit in contrast to other regen designs which are problematic to replicate. (See: www.vk2zay.net/article/128)
In the circuit the differential pair
@Sean O'Connor wrote:
I think another method, that I haven't tried in a regen, is to swap the emitter and collector of the BJT for lower gain. That might also give smooth regen control. LTspice can correctly simulate a BJT in this mode.
I used a BJT with collector and emitter swapped as part of a 1.2-volt AGC circuit. The "reversed" BJT acted as a variable resistance, forming the bottom half of a voltage divider. When I used the BJT in "normal" mode, the gain was too high and the transistor reacted too suddenly to base bias changes. In contrast, in "reversed" mode, the BJT reacted more slowly. This was confirmed both in LTspice and in hardware. Some notes here: https://qrp-gaijin.blogspot.com/2015/09/12-volt-audio-based-agc-for.html .
For your amusement, here is a differential pair/cross-coupled oscillator that is oscillating with one of the BJTs connected in reversed fashion. It might have smoother regen control due to the lower gain.
The above circuit wouldn't oscillate at supply voltages of 1.2 volts or lower, indicating that the gain is indeed lower in this configuration.
The circuit will also oscillate if the Q2 C/E connections are normal, but the Q1 C/E connections are swapped.
The circuit has difficulty oscillating, however, if the C/E connections on both Q1 and Q2 are swapped, probably because the gain is too low overall. But it can be made to oscillate under some circumstances, if the L/C ratio is high enough.
In this configuration, you can see that the waveforms quickly become distorted, I think due to the fact that the transistors are quickly becoming saturated at low voltages, which is a characteristic of using BJTs in this reversed fashion.
So there we have it -- an "upside-down" cross-coupled oscillator! Is it useful -- who knows? 😀
Here is an improved version of the noisy regen which consists of an AC-linked differential pair Q-Multiplier in which the regen control is done by changing the base bias current of both Q1 and Q2. R5 is the course regen control, R6 is the fine regen control. The current mirror detector circuit Q3-Q5 can also be connected at the top of the tank circuit L1/C4. Circuit has been drawn in DigiKey's Scheme-it at https://www.digikey.nl/en/schemeit/project.
Advantages of this circuit:
Q1 and Q2 don't have to be equally matched, because each BJT has its own independent bias. This will give a smooth and backlash-free regen control;
The base-collector junctions of both Q1 and Q2 will always be reverse biased, which reduces loading of the tank circuit L1/C4 and causes less phase noise when the Q-Multiplier is oscillating while detecting SSB and CW;
The tank circuit L1/C4 has been lossely conected to the base of Q2 by using a small coupling capacitor C5, which also reduces loading of the tank circuit and decreases phase noise.
While the Noisy Regen may be a poor AM detector, but this circuit is very suitable for using as a synchronous FM detector to detect wideband FM broadcast stations. See https://www.electroschematics.com/small-fm-receiver/ "Simple FM Receiver Circuit".
Circuit diagram (Source: https://www.electroschematics.com/small-fm-receiver)
If you measure the collector emitter voltage of the transistors in the noisy regen circuit it is about 0.6 or 0.65 volts which is above the collector emitter saturation voltage of small signal transistors. That is how the circuit can still work, even with what you could say is a very odd biasing technique.
As you get nearer the staturation voltage the gain of the transistor falls. And actually often the saturation voltage is defined as the point where the gain of the transistor (Hfe) has fallen to 10% of its normal value.
Even there this is some gain.
It might be interesting to build a regen with a transistor operating in that region. It's possible you might directly get smooth regeneration control down there. FETs I seem to remember have a linear response mode at very low source dain voltages but I never got that to work properly, if you could find that zone and there was enough gain you would get very smooth regeneration control.
I suppose the follwing noisy regen circuits would work and of course you get to keep the very easy band switching capability of the basic noisy regen circuit:
You can clearly see Q3 and Q4 form a current mirror at DC if you view L1 as a short. You can possibly omit C2 especially above 2 or 3 MHz. The disadvantage of the circuit is the LC resonant tank L1 C1 is directly connected to the base input impedance of Q1 which might only be 5 or 10k. And the tank is directly connected to a number of semiconductor junctions which is not good for frequency stability and SSB reception. You could put a bypass capacitor (100n) between the collector and emitter of Q3 but I left it out to avoid any RC constants that might cause any squealing effect.
Sean O'Connor wrote:
I agree with the above, except for the last point. In my regenerative superhet, I found that even taking the signal off of the emitter yielded poor performance, which was most easily observed by trying to listen to CW above threshold, which was impossible due to very noisy oscillation above threshold, instead of the pure sine wave audio tones that you should hear above threshold. I also noticed that just below threshold, there was an unacceptable increase of noise instead of the expected seashell-like sound caused by narrowing the bandwidth. And there was also other highly annoying and troublesome behavior like a tendency to motorboat on strong signals (although I later found out that poor voltage regulation probably also contributed greatly to the tendency for motorboating).
For a long time this puzzled me, until I finally decided to take the RF signal from a low-impedance capacitive tap on the tank. At that point, behavior was vastly improved and I could finally narrow the bandwidth below threshold and listen to clean CW above threshold.
My guess is that in my circuit, my detector stage was probably causing highly variable loading, which when connected to the emitter of the differential-pair oscillator, was probably dynamically affecting the regeneration level and hence causing unwanted excessive noise at threshold.
The really annoying thing about debugging this is that it was quite easy to misinterpret the excessive threshold noise as being a natural property of the regenerative amplifier at critical threshold, which was wrong -- the excessive threshold noise was due to bad interaction with the following detector stage.
Of course, taking the signal off of a low-impedance capacitive tap on the tank gives a much lower signal level for the detector to work with. Because my circuit was a superhet, I could simply add more IF gain in order to provide enough signal for the detector to work with. For a straight regen, you might need a large antenna (or even double regeneration) to get enough signal for the detector.
Anyway, you can read about my experiences here: https://www.theradioboard.org/forum/main/comment/7e06906d-2163-430d-94bc-5161e33f2699?postId=62cd4c7ddec9c100122bee3b .
Interesting circuit. Has anyone tried it?