sites.google.com
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? 😀