Transistor Amplifier Circuits Unit 9 – Direct Coupling
Exercise 1 – Direct-Coupled Amplifier DC Operation
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to determine the dc operating
conditions of a direct-coupled, two-stage amplifier by using measured values. You will verify
your results with a multimeter.
DISCUSSION
- The direct-coupled two-stage amplifier circuit consists of two transistors configured as
common emitters. The first stage, Q1, uses an NPN transistor and the second stage, Q2, uses
a PNP transistor. - The output of Q1 (the collector) connects directly into the input of Q2 (the base). The
collector voltage of Q1 is equal to the base voltage of Q2. (VC1 = VB2) - The emitter voltage (VE2) is about 0.6 Vdc more positive than the base voltage (VB2) when
the base-emitter junction of Q2 is forward biased. - The collector voltage (VC2) is less positive than the base voltage (VB2) since the base-
collector junction is normally reverse biased. - The first stage has a voltage divider network to set the dc bias. The collector voltage of this
stage sets the base voltage of the second stage. - The second stage does not use a voltage divider to establish the dc bias; the resulting bias is
more sensitive to temperature changes. - The emitter resistor in the second stage generates feedback that counteracts dc bias drift due
to temperature. But, it is not as effective as voltage divider bias.