Build Your Own Transistor Radios: A Hobbyist's Guide to High-Performance and Low-Powered Radio Circuits by Quan Ronald

Author:Quan, Ronald
Language: eng
Format: epub, azw3
Publisher: McGraw-Hill Education
Published: 2012-12-11T05:00:00+00:00

FIGURE 14-11 A differential-pair oscillator mixer circuit.

If RF gain reduction is needed, an emitter capacitor CE can be replaced with a series capacitor and resistor combination (RC), and the transconductance of Q3 is lowered.

Conversion Gain

Conversion gain is usually the ratio of the voltage amplitudes of the output IF signal to the input RF signal. When the conversion transconductance is known, the conversion gain is just

where R1 is the equivalent load resistance (usually) in a parallel LC tank circuit. If the mixer’s output current is connected to a tapped transformer, then the turns ratio must be taken into consideration. For example, many IF transformers have a turns ratio of 3:1 in the primary winding. If the IF transformer has an equivalent parallel resistance of 200 k, then the resistance at the low-side tap is (1/32) × 200 k, or 22.2 k = R1. Also note that the output signal at the secondary winding of the IF transformer will be lower than at the primary winding owing to the step-down ratio.

Before I end this chapter, I offer Table 14-5, which lists various conversion transconductances of different devices for comparison. For the one-transistor mixer, assume that the driving voltage at the base-emitter junction is above 125 mV peak, which translates to a conversion transconductance of about 90 percent of the small-signal transconductance of a bipolar transistor.

As can be seen in Table 14-5, the two vacuum tubes, 1R5 and 12BE6, which were used in superheterodyne radios, have conversion transconductances very close to that of a bipolar transistor running at a collector current of 10 μA. This fact will give you a hint as to how the ultralow-powered superheterodyne radios of Chapter 9 were designed.

TABLE 14-5 Conversion Transconductances of Transistors, Vacuum Tubes, and MOSFETs

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