2K12 Regenerative Receiver
A century ago in 1912 radio receivers were still rather primitive devices.
Very high transmitter power levels were required to obtain a range of even 30 or 40 miles. All this changed when a young man named Erwin Armstrong found out how to obtain tremendous amplifier gain out of a single triode tube using regeneration. This was an important discovery that gave millions of people their first chance to listen to broadcast radio at an affordable price. The demand for radios based on Armstrong’s circuit was so great that manufacturers of the time found it difficult keep up.
Today you can still make a great radio using the regenerative effect but to do so you must recreate important aspects of the amplifying device used in Armstrong’s circuit using modern semiconductors.
The Armstrong circuit
The amplifying device used by Armstrong was the triode tube (Figure 1). The flow of current between the heated cathode and the anode is controlled by the grid voltage. The key aspects of triode behaviour for the regenerative circuit are it’s high input impedance of several hundred kilo-ohms at the grid and the decrease of gain with increasing input signal amplitude that stabilizes the behaviour of the circuit.
A simple example of the Armstrong regenerative circuit is shown in figure 2. The L1 and C1 form a resonant circuit which provides basic frequency selection of signals from the antenna. The signal is then feed to the triode grid through the gird leak resistor R1 and capacitor C2. The resulting variations in grid voltage create a variation in the anode current going through L2.
L2 is the so called tickler coil which is mechanically arranged to provide an adjustable amount of magnetic coupling with L1.
L2 can be connected to provide either negative or positive feedback to the system by swapping it’s connection leads. Negative feedback reduces gain and flattens out the frequency response curve. Positive feedback increases gain and exaggerates the frequency response curve. Of course Armstrong was after the increase in gain given by positive feedback and as a bonus the frequency response (determined by the LC circuit) was greatly exaggerated giving much improved selectivity. If too much positive feedback is used the circuit will start to oscillate. Maximum gain and selectivity are obtained when the circuit is adjusted to exactly the threshold of oscillation. It is important that the triode is biased in such a way that it’s gain decreases with increasing AC input signal voltages. This is done by keeping the anode voltage below about 25V and using a grid leak resistor to keep the grid bias voltage close to zero. If the gain does not decrease with an increasing input signal voltage then the transition into oscillation as positive feedback is increased will not be smooth. The circuit will suddenly burst into oscillation as positive feedback is increased and won’t stop oscillating until the amount of positive feedback is greatly decreased, making the receiver impossible to operate.
Detection of the radio signal amplified by the Armstrong circuit is obtained by variations in the input impedance of the grid when it is biased close to zero volts. As the grid voltage becomes more positive it’s input impedance drops, resulting in a rectifying action which demodulates AM radio signals.
Emulating a triode
The most difficult property of the triode to copy using semiconductors is the decrease in gain with increasing input signal level. It is possible to adjust the operating points of many types of FET and dual gate MOSFET to give decreasing gain with increasing input signal. However the results are unreliable and depend very much on individual device parameters. Ordinary transistors such as the 2N2222 are nearly impossible to bias into a suitable operating point. The universal solution is to use two active devices in a differential pair configuration. The underlying physics guarantees that the gain will decrease with increasing input signal amplitude. If you want to use unmatched devices then the AC linked differential pair circuit is the best solution (See figure 3).
With a triode the input impedance at the gird is high enough that there is no significant loading of the LC circuit that would reduce selectivity. Also there is very little variation of input capacitance with input level that can likewise reduce selectivity. Ordinary transistors have low input impedance and show a wide variation of input capacitance with input level. FET and MOSFET devices have a much higher input impedance but still show some variation of input capacitance with input level. The solution to both the input impedance and input capacitance problem is to do an impedance transformation using a capacitor divider circuit as shown in figure 4.
The final aspect of triode behaviour to emulate is the rectifying action at the gird that is used to demodulate AM radio signals. It is better to use a separate AM detector such as the transistor square law detector shown in figure 5.
A practical circuit
Putting all the pieces together a suitable circuit using standard transistors is shown in figure 6. The amount of positive feedback is controlled by the potentiometer R14 which changes the gain of the differential pair Q3 and Q4. The very low input impedance of the transistor Q3 connected to the LC circuit (L1 and C13) means that a high ratio capacitor divider comprised of C9 and C10 is required to avoid excessive loading. The high gain of standard transistors compared to that of FET and MOSFET devices offsets the reduction of gain caused by the divider circuit. R10 is a stopper resistor that prevents parasitic oscillations in the VHF and UHF range. It should be placed as close to the collector of Q3 as possible. L3 is radio frequency choke of about 100uH and can be constructed by winding 100 turns of fine wire over about ¼ of inch on a pen barrel. The inductance of L1 and the capacitance of the variable capacitor C13 in parallel with the fixed capacitance C10 determine approximately the frequency range covered. You can use a coil inductance calculator from the internet with the well known LC resonance frequency formula to figure out exactly your requirements or you can simply experiment. The number of turns to use for the fixed tickler coil L2 is really a matter for experimentation but should only be a fraction of the number of turns on L1. Q1 is used as a transistor square law detector and Q2 acts as an output buffer.
The audio output from the receiver should be feed to a high gain audio amplifier. It is important to power the receiver from a regulated power supply for maximum stability. Switch mode power supplies are not suitable because they generate too much RF interference.
By emulating as many of the advantages of the triode as possible you can create an effective regenerative radio receiver using common components. Because regenerative radio receivers require precise adjustment of both positive feedback and frequency, careful mechanical construction of the receiver is well rewarded. It is a good idea to put the receiver in a shielded case to avoid hand capacitance effect which can make tuning difficult. A long antenna is not required, a random wire antenna a few feet long will pull in a large number of stations. The useful frequency range of the circuit show is 500 KHz to 30MHz.