Q Multiplier Circuits

Q Multipliers

Q multipliers were commonly used in amateur radio circuits in the past, especially in shortwave receivers. They were used to improve the selectivity and sensitivity of the receivers, making it possible to receive weaker signals and to distinguish between signals that were close together in frequency.

Here are some specific examples of amateur radio circuits from the past that used Q multipliers:

• The Heathkit AR-3 receiver, which was a popular shortwave receiver in the 1950s, had a built-in Q multiplier.
• The Drake 2B receiver, which was another popular shortwave receiver in the 1950s and 1960s, had a separate Q multiplier unit that could be added to the receiver.
• The Collins KWS-1 transmitter, which was a popular amateur radio transmitter in the 1960s and 1970s, used a Q multiplier to generate the SSB signal.

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Operation

By cancelling out the resistance and other losses in an LC (aka LCR) circuit its Q (voltage magnification at resonant frequency) is greatly increased.

You can do this using either a small controlled amount of positive feedback or by using a negative resistance circuit.

As you increase the amount of positive feedback the Q increases until, just at the threshold of oscillation it reaches a maximum. Applying even more positive feedback the circuit starts oscillating and the Q starts decreasing.

Noise and in particular phase noise in the sustaining amplifier places a limit on how much Q multiplication is actually possible.

Unwanted phase shifts in the sustaining amplifier also reduce the amount of Q multiplication available and cause shifts in the exact resonance frequency of the overall circuit.

The sustaining amplifier or negative resistance source must by some way or another have decreasing gain with input signal amplitude. Since increasing gain equates to increasing positive feedback with input signal strength resulting in a control problem. The circuit suddenly pops into strong oscillation as you approach the threshold of oscillation as positive feedback keeps increasing with input signal strength leading to a further increase of positive feedback, leading to....

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Practical Circuits

The differential pair meets all the necessary requirements for a Q multiplier.

There are a number of simple circuits that rely the collector emitter saturation voltage of BJT transistor being lower than the base emitter voltage switch on voltage.

If the saturation voltage is 0.2V and the switch on voltage is 0.65V then there is about 0.45V of voltage freedom at the collector.

Figure 1.

Figure 1 shows the basic configuration. Since the input impedance at the base of Q2 is quite low it is better to have an impedance matching winding for the tuned circuit rather than directly connecting the tuned LC circuit directly to the collector of Q1. Of course some input signal is coupled to the LC circuit in some way and extracted. It is generally sufficient that the 2 transistors are of the same type and batch. Exact matching of the transistors is not required.

Figure 2.

Figure 2 shows a circuit with buffered output. However a disadvantage is the Miller effect feeds back to the base of Q2 lowering its input impedance.

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Figure 3.

The circuit in figure 3 prevents the Miller effect and gives better buffering of the LC circuit than figure 2.

Figure 4.

The circuit can be rearranged to supply controlled current to the collectors of the transistors rather than the emitters as in figure 4.

Figure 5.

In figure 5 transistors Q1,Q2,Q3 form a current mirror with Q4 at DC.

At RF Q4 acts as "square law" type AM detector. If an antenna is very loosely coupled to the LC circuit you have created a regenerative radio circuit.

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