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How to Bias a BJT Class A Amplifier

A Class A bipolar junction transistor amplifier consists of a single transistor, two biasing resistors, a pair of capacitors and a battery or other DC power source. The BJT̵7;s amplifying ability begins only when the input signal exceeds about .65 volts; below that value, the transistor is off. In addition, the transistor works only on positive voltages; it ignores the negative voltage half-cycle of an AC waveform. The biasing resistors turn AC input signals such as audio and radio frequencies into positive voltages; the transistor then amplifies these biased signals with very good fidelity.

Things You'll Need

Class A BJT amplifier schematic
Pencil
Calculator

Instructions

- 1
  Locate the resistor that connects between the circuit̵7;s ground and the transistor̵7;s base lead. Label it as ̶0;R1.̶1;
- 2
  Locate the resistor on the schematic that connects between the positive DC supply and the transistor̵7;s base lead. Label it as ̶0;R2.̶1;
- 3
  Calculate the ohm value for R1:
  
  R1 = Vs / ((110 * .002)/B)
  
  Vs is the supply voltage and B is the transistor̵7;s beta, or current amplifying factor. The transistor̵7;s data sheet has its beta specification. A typical transistor has a beta of 100; you can use this number in the calculation.
- 4
  Calculate the ohm value for R2 by multiplying R1̵7;s value by 10.

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