AF PREAMPS
by Harry Lythall - SM0VPO

Although I have called these circuits Audio Frequency (AF) pre-amplifiers, they will often work quite well up into the RF spectrum, sometimes into the VHF region also. You may have to place 100pf or more between the base/emitter of each transistor if you are using these circuit for high gains at low frequencies. Typical supply voltage is from 3 to 30 volts for all circuits.

Simple Single Transistor

This first circuit is about the simplest small-signal amplifier I know and is one of those building blocks that use for just about all occasions.

C1 and C2 govern the low frequency response. R2 is selected for a standing DC voltage of half the supply voltage at the collector. R1 controls the amplifiers gain and may be a short-circuit for maximum gain. With R3=150R then this circuit will form a lovely HF / VHF / (UHF?) preamplifier with as much as 15dBv gain using a BFR90 transistor. The input and output could be coupled to the center-tap of tuned circuits to form a tuned RF amplifier. 50 ohm couplings to the tuned circuits would be typically 10% of the number of turns of wire used.

Simple Two Transistor

Now this circuit is a little more controlable and is usefull for frequencies up to about 50MHz. For Audio Frequencies you can get voltage gains as high as 25dBv.

The gain is preset by 1+R5/R3. If there is half-suply voltage present on the previous circuit driving this amplifier then R1, R2 and C1 can all be omitted and TR1 base connected directly to the input. You may also need a 47K resistor (or smaller) between TR2 base/emitter (R4) if TR1 has a high leakage.

Simple Three Transistor

This circuit is identical to the two transistor circuit above, but the output is buffered. This circuit is quite capable of driving lower impedance loads, such as "walkman" (freestyle) stereo headphones (or antennas?). R7 is typically 270R and C3 would be as much as 470uf or more. All other components are the same as the two transistor circuit.