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Armstrong Q multipliers are simple to implement and allow good matching of the tuned circuit to the sustaining amplifier's input and output impedance's, the tuned circuit can retain maximum Q.
It also allows the sustaining amplifier to operate at a low phase shift (lag or lead), which also keeps the effective Q high.
The Q of the tuned circuit is then boosted above its natural level by the sustaining amplifier feeding back enough energy to cancel out the losses: Q Multiplication.
Unlike most other oscillator circuits there isn't an intrinsic way to limit the strength to which oscillations may build-up in the basic Armstrong circuit, some care in design is indicated or plastic transistors may "pop" for example.
One of the simplest safe Armstrong Q multipliers is shown in Figure 1.
Figure 1. Armstrong Q multiplier. Generally C2 should be omitted. It can be included to reduce noise, including potentiometer noise though in may introduce slightly unwanted control effect or in the worst case super-regeneration. If the circuit isn't working try reversing the connections to the coil connected to the collector. The phasing must be correct to get positive feedback.
Even though the transistor are biased in an odd way, they still operate within a typical datasheet defined active region. The cost of operating at lower voltages is slightly reduce transistor gain, increased collector base capacitance and somewhat lower collector output impedance.
Q1 and Q2 should be of the same type since they configured together as a differential pair, matching is generally not needed. The decreasing gain versus input signal strength of a differential pair makes control of Q multiplication easy.
The collector coil typically has 1/5 to 1/10 the number of turns on the tuned circuit coil. The base coil 1/5 to 1/15.
You also need some way to couple a signal in and out of the tuned circuit such as a low value (a few pf) capacitor link. You can connect one end of the tuned circuit to ground.
You can also obtain a buffered output from the collector of Q2 as shown in figure 2.
Figure 2. Q multiplier with buffered output. You can also create an Armstrong oscillator with buffered output by choosing a suitable value for R1 and connecting it straight to ground omitting R2/C2.
To convert a Q multiplier to a regenerative radio you need a constant input impedance AM detector, see figure 3.
Figure 3. A constant input impedance AM detector. You can replace the audio transformer with a 1k resistor or 32 ohm headphones. You can increase the headphone output by reducing R3 to say 10k. The LED provides a signal strength display and could be omitted. The pickup coil is a few turns around the tuned circuit inductor at a ratio of about 1/7 to 1/10 or it could be the buffered output from figure 2.
You can combine the circuits in figures 2 and 3 however the audio output may be a little lower because it depends on the current through the poteniometer R2.
Figure 4. Regenerative radio from combined Q multiplier and AM detector.
Other simple Armstrong type Q multipliers are based on FETs.
Figure 5. Green LED Regen. The negative bias for the FET is supplied by the green LED, C1 is a 3pf NPO capacitor or a "gimmick" capacitor made from twisting a pair of insultated wires together.
Figure 6. Using the K596 capacitor microphone FET the circuit can be simplified. The FET contains a 25 meg resistor from gate to source and has a low Idss removing the biasing requirement.
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