CHAPTER 5 - Amplifier Design
208
5.3 Amplifier Design Stages
Amplifier design comprises of three main stages
o Biasing
o Stabilisation
o Impedance Matching
These are explained in detail in the following subsections and in their respective video tutorials.
5.3.1 Stage 1 - Biasing
The first step when designing an amplifier is choosing the appropriate bias voltages for the
selected active device. These will be dependent on the target application and the specifications of
the device. For example, if we are designing a mobile phone amplifier we will have to select voltages
that may be supplied by its battery. We will also need to choose the bias voltages so as not to
exceed the maximum voltage and current ratings of the transistor.
In Figure 5.3-1 one of the most primitive forms of biasing is illustrated. In this case we are
independently feeding bias voltages to our base and collector terminals. Although this would work,
there is no DC feedback between collector and base terminals which means that, should the
collector current change, the base voltage has no means of "adapting" to this change.
Figure 5.3-1 Primitive Biasing for BJT
In Figure 5.3-2 a different bias topology is shown which provides dc feedback. In this
network, should the collector current increase, the voltage at the collector terminal of the transistor
will decrease, due to a higher voltage drop across the resistor R
C
. This, in turn, will decrease the
voltage difference between base and collector terminal V
BC
thereby decreasing the base current.
C
E
B
CC
V
C
R
2 B
R
CC
V
1 B
R
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