BJT (Bipolar Junction Transistor) is usually used in transistor circuits for amplification of voltage or current. It operates with both AC and DC voltages. AC signal is the signal which is amplified and the DC voltage is used for biasing of the transistor. Since the the components behave differently for DC and AC signals, therefore different methods are used for the analysis of the circuit.
The simplest and most widely used method is using the DC and AC equivalents. In this method the original amplifier circuit is converted to DC and AC equivalents and then the voltages and currents in both the circuits are calculated using KCL( Kirchoffs current law) or KVL (Kirchoffs voltage law) and then using the superposition theorem the final values are obtained.
Additional information
KCL: The algebraic sum of currents in a network of conductors meeting at a point is zero.
KVL: The sum of the emfs in any closed loop is equivalent to the sum of the potential drops in that loop.
The simplest and most widely used method is using the DC and AC equivalents. In this method the original amplifier circuit is converted to DC and AC equivalents and then the voltages and currents in both the circuits are calculated using KCL( Kirchoffs current law) or KVL (Kirchoffs voltage law) and then using the superposition theorem the final values are obtained.
Additional information
KCL: The algebraic sum of currents in a network of conductors meeting at a point is zero.
KVL: The sum of the emfs in any closed loop is equivalent to the sum of the potential drops in that loop.
Superposition theorem: For a linear system the response (voltage or current) in any branch of a bilateral linear circuit having more than one independent source equals the algebraic sum of the responses caused by each independent source acting alone, where all the other independent sources are replaced by their internal impedances.
Simple BJT amplifier circuit
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DC Equivalent
Steps:
- All the capacitors in the circuit are replaced by an open circuit.
- The AC sources present in the circuit are grounded.
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With reference to the above given circuit, the capacitors Cin and Cout becomes open and the Input AC source is grounded.
Simplified circuit :
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AC equivalent
Steps:
- All the capacitors are replaced by short circuit.
- The DC sources present in the circuit are grounded.
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With reference to the original amplifier circuit, the capacitors Cin and Cout becomes short and the DC source Vcc gets grounded.
Simplified circuit:
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Since the original circuit is converted to its AC and DC equivalent, hence it is ready for analysis.
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