15-05-2013, 03:06 PM
BJT Small Signal Model and AC ampliers
BJT Small Signal.pdf (Size: 184.99 KB / Downloads: 63)
INTRODUCTION
We calculated the DC behavior of
the BJT (DC biasing) with a sim-
ple large-signal model as shown. In
active-linear region, this model is
simply: vBE = 0:7 V, iC = iB.
This model is sucient for calcu-
lating the Q point as we are only
interested in ensuring sucient de-
sign space for the amplier, i.e., Q
point should be in the middle of
the load line in the active linear re-
gion. In fact, for our good biasing
scheme with negative feedback, the
Q point location is independent of
BJT parameters. (and, therefore,
independent of model used!)
A comparison of the simple model
with the iv characteristics of the
BJT shows that our simple large-
signal model is very crude and is
not accurate for AC analysis.
BJT Amplier Circuits
As we have developed dierent models for DC signals (simple large-signal model) and AC
signals (small-signal model), analysis of BJT circuits follows these steps:
DC biasing analysis: Assume all capacitors are open circuit. Analyze the transistor circuit
using the simple large signal mode as described in pp 57-58.
AC analysis:
1) Kill all DC sources
2) Assume coupling capacitors are short circuit. The eect of these capacitors is to set a
lower cut-o frequency for the circuit. This is analyzed in the last step.
3) Inspect the circuit. If you identify the circuit as a prototype circuit, you can directly use
the formulas for that circuit. Otherwise go to step 3. 3) Replace the BJT with its small
signal model.
4) Solve for voltage and current transfer functions and input and output impedances (node-
voltage method is the best).
5) Compute the cut-o frequency of the amplier circuit.
Several standard BJT amplier congurations are discussed below and are analyzed. Because
most manufacturer spec sheets quote BJT \h" parameters, I have used this notation for
analysis. Conversion to notation used in most electronic text books (r, ro, and gm) is
straight-forward.