Electronic – Identifying strange amplifier types of amplifiers in a 1979 CB radio

amplifierbjtradio

I'm trying to fix an old Midland 3001 CB radio and I came across a few strange types of amplifiers in the receive stage of the radio. I was wondering if anyone could identify the type of amplifier used.

In the screenshot the direct input from the antenna is coupled through C101 on the left I believe. Then there are some diodes which I assume are used to protect the receive stage. The L101 is a variable transformer that couples the signal to the transistor Q101.

  • What kind of amplifier circuit is the Q101 transistor involved in (is it common emitter, common collector, or something else entirely)?
  • What is L102 doing at the output (I believe it is a tunable LC
    filter)?

I have also attached the full schematic diagram for the radio.

The amplifier in question
Midland 3001 schematic

Best Answer

D101 along with D105 do protect the receiver input from huge transient events, as OP surmises. They also limit amplitude of the transmitted signal going into the receiver. But D103,D104 are DC-biased with current from the RF GAIN control, to attenuate merely large input signals while receiving.

Common-emitter RF amplifier transistor Q101 has resonators (27 MHz) at both input (L101) and output (L102). C101 (33pf) resonates with L101 @ 27 MHz with fairly low-Q. A small-value neutralizing capacitor (C104) keeps this amplifier from oscillating. It compensates for collector-to-base capacitance inside the transistor.

L102 is a tuned circuit (27 MHz), part of a double-tuned resonator along with L108, top-coupled with a small coupling capacitor (C107, 3pf). This double-tuned resonator has a bandwidth wide enough to pass the whole CB band on to the down-converting mixer.
A noise blanker (NB) module (possibly an optional addition?) also takes its input from this double-tuned resonator.