Page 239 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

IL ) 1 ~ ~ ut
Therefore, the conversion gain is 0.00311 mho x 150,000 V = 466. This would
mean that an RF signal is amplified 466 times and converted to an IF signal of
generally lower frequency. For example, if the RF signal is 1 mV peak to peak at
600 kHz and the local oscillator provides a 1,055-kHz signal at 78 mV peak with a
100-IJA DC collector current for the one-transistor mixer and a 1S0-V load at 455

kHz, the output IF signal at 455 kHz will be 466 mV peak to peak.
Also, it should be noted that because the RF signal is generally small, the worst
case or minimum input resistance into the base of the mixer is ~/gmQ' which is the
small-signal input resistance at the (quiescent) DC collector current.

Differential-Pair Mixer
The differential-pair transistor amplifier is a building block to a true multiplying

circuit. A double-balanced multiplier performs the operation of literally multiplying
two signals without leaking through any signals from the inputs (e.g., RF signal
and/or oscillator signal) to the output. Normally, this "true" multiplying circuit will
require at least three or four differential-pair amplifiers. For example, see the data
sheets and schematic diagrams to the MC1494 and the MC1495.
For this chapter, we will just look at a simple differential-pair mixer, which will

provide a multiplying action but also will leak through the input signals, the RF
input signal, and the oscillator signal at its output. However, the input signals
generally can be filtered out from the output of a differential-pair mixer.
A differential-pair mixer usually consists of at least three transistors. See Figure

14-8. In Figure 14-8, the oscillator signal will be connected to the base of transistor
Q1, and Q3's collector will be providing the modulating current that controls the
signal current at the output of Q1 (or Q2). Generally, the top transistors are in a
"limiting" mode, which means that the oscillator voltage across the bases of Q1 and
Q2 is sufficiently high in amplitude (e.g., >300 mV peak) to cause Q1 and Q2 to
generate a square-wave output current. At an amplitude level of about 1 volt peak
to peak sine wave to the base of Ql, the output current of Ql or Q2 does indeed

resemble a square-wave signal. Figure 14-9 shows the output waveform of Ql for
various levels at the base of Q1. Note: In practice Rl is replaced with an IF filter.