VFO Variable Gain Amplifier

Variable Amplifier

This circuit can be used as the first amplifier of the VFO amplifier chain, or as the BFO amplifier (with a 455kHz IF can replacing the broadband coil) to provide variable injection levels for fine tuning weak signal reception.

The 100K variable resistor can be mounted on the front panel to allow adjustment at any time. It is also useful to mount the LED from this amplifier on the front panel to remind you of how much injection you're using.

I would fire up the receiver and have no signals, until I realized the VFO amplifier LED was very dim. The brightness of the LED indicates injection level. Very bright - highest injection, low brightness - lower injection levels.

Higher injection levels give greater conversion gain which equals better sensitivity. Lowering the injection level kept strong signals from modulating the noise level, an aid when trying to copy a weak signal right next to a very strong signal. I found it helpful during contesting and working DX.

An additional benefit was that varying the bias on the first amplifier also changed the VFO frequency 1 to 2 kHz. This made it easier to use variable capacitors and vernier drives of moderate quality. I was able to keep CW signals in the bandwidth of very sharp crystal filters. This variable bias served as a mini-bandspread control at the upper range of the injection level.

The circuit also shows an 18pf capacitor off the output to feed a frequency counter. In most cases, no additional amplification was necessary. When I did need to raise the level, I used the MMIC like the one in the Beginner and Experimenter's Receiver. A 74HC4049 CMOS Hex Buffer chip was used to make a square wave for the input to the frequency counter. Use a one megohm resistor to ground at the input of the hex buffer to protect the chip. Ground all unused hex buffer inputs. A 4049 is used because the input to the hex buffer can exceed the supply voltage of the chip and not burn up. Use the same supply voltage of the frequency counter for the 4049 to keep from burning up the input of the frequency counter chip.

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Last Update: 01/21/2000
Web Author: David White, WN5Y