JFET Regenerative Radio Receiver
JFET Regenerative Radio Circuits
The careful use of JFET devices in regenerative radio design can bring many benefits. JFET’s have much lower noise than BJT transistors and their high input impedance allows them to be directly connected to a resonant LC circuit without the impedance matching that BJT’s require. Historically JFET devices have had lower transconductance gain than BJT’s. However many modern devices such as the 2SK222, BF862 , 2SK170 and IF9030 have very good gain characteristics and exceptionally low noise figures. Besides noise and gain there are 2 other figures of merit you need to look at. The first is the IDSS. This is the amount of current that will flow through the JFET when there is zero bias voltage at the gate of the JFET. The value should be high enough that the JFET has good gain characteristics without having excessive thermal dissipation. Any value between 2mA and 20mA is usually ok.
The second important figure of merit is the input capacitance Ciss. A high input capacitance will add extra capacitance to the LC resonant circuit. More importantly it will increase the amount of frequency drift with temperature change. For low frequency reception a high input capacitance is not a big problem, for higher frequency work you might consider using a JFET with a low input capacitance. Additionally if frequency drift is still a problem you can connect the LC resonant circuit to the JFET gate via a tap halfway down the inductor to reduce the frequency drift by 4, or 2/3 of the way down to reduce the frequency drift by 9.
A simple but effective regenerative radio circuit is shown in Figure 1.
The 2 JFET devices are arranged as an AC-linked differential pair. This is important because the gain of a differential pair decreases with increasing input level. If this gain requirement is not met then the regenerative receiver will pop into oscillation as the amount of regeneration is increased and not stop oscillating until the regeneration control is turned far back. L1 is the regenerative tickler coil which supplies positive feedback to the LC resonant circuit C2 and L3. If L1 is connected the wrong way around it will incorrectly supply negative feedback instead. You will need to create a mechanical positioning system to move L1 close to L3 to control the amount of regeneration. The exact number of turns required for L1 is best determined by experiment. L2 is a 220uH radio frequency choke (RFC), for reception frequencies lower than 5MHz it's value should be increased to a few mH or 1k resistor should be placed in parallel with it to reduce any possible effects of self-resonance. Ideally L2 should be self-shielded to reduce the chances of AC hum pickup. Q1 forms a very sensitive transistor square law AM detector.
Figure 2 shows a very similar circuit however here R1, R2 and C1 have been replaced by a resonant LC circuit C1 and L4. Ideally the variable capacitors C1 and C2 should be ganged together. Because of the low impedance environment the resonant circuit C1 and L4 is very broadly tuned and does not require exact alignment with resonant frequency of C2 and L3. However inclusion of a trimmer capacitor in parallel with C1 might be a good idea. The circuit in figure 2 has slightly higher gain than the circuit in figure 1 and may have some advantages with higher input capacitance JFET’s.
If you use JFET’s intended for VHF or UHF work you may need to include stopper resistors or ferrite beads in the gate and drain of J1 to prevent parasitic oscillations. It is usually easy to detect parasitic oscillations because the brightness of the red LED D1 will change as you move your fingers near the circuit.
Mixed Technology Circuit
Figure 3 shows a mixed JFET - BJT regenerative circuit. The amount of regeneration is controlled by adjusting the potentiometers R3 and R6. Increasing the voltage at the base of Q2 steers signal current from the source of J1 to the regenerative feedback coil L2.
JFET's allow you to create very high sensitivity regenerative radios. The low phase noise of the resulting circuits also gives you extremely high selectivity. When used with a good voltage regulator the resulting circuit has high frequency stability. One possible way to improve frequency stability further would be to create a small temperature control circuit to hold the JFET's at constant temperature. You can easily experiment with different JFET's to see which give the best trade-off between low noise, selectivity and frequency drift. Generally the frequency drift is very low anyway once the circuit has warmed up for a few minutes. There are some great modern JFET devices currently available and a JFET regenerative receiver is unique way to explore their characteristics.