Analogue Electronics II: BJT Amplifiers

AC Quantities and Amplifier Operation

• The biasing of a transistor is a dc operation, used to establish a Q-point for variations in current and voltage to occur in response to an ac input signal.
• Small-signal amplifiers are designed to handle small ac signals from sources like antennas or microphones.
• AC quantities are represented by uppercase subtitles, while instantaneous quantities are represented by lowercase letters and subscripts.

The Linear Amplifier

• A linear amplifier provides amplification of a signal without distortion, resulting in an exact replica of the input signal.
• A voltage-divider biased transistor with a sinusoidal ac source and capacitive coupling is an example of a linear amplifier.
• The coupling capacitors block dc and prevent the internal source resistance and load resistance from changing the dc bias voltages.

Transistor Ac Models

• The five r-parameters commonly used for BJTs are resistances internal to the transistor, represented by italic lowercase letter r with a prime.
• The r-parameter transistor model can be simplified or generalized, depending on the application.

Use of Capacitors in Amplifier Circuits

• Capacitors store electrical charge, have impedance that decreases with frequency, and can block dc between stages or couple desired ac signals between stages.
• Coupling capacitors are used to pass ac signals between stages, and bypass capacitors are used to bypass unwanted ac signals to ground.
• Capacitors can be designed to have low impedance at high frequencies, making them useful for signal coupling.

Calculating Amplifier Performance

• Voltage gain (Av), input impedance (zin), output impedance (zout), and current gain (Ai) are important performance metrics for amplifiers.
• These metrics can be calculated using the following formulas:
  • Av = vout / vin
  • zin = vin / iin
  • zout = vout(OC) / iout(SC)
  • Ai = iout / iin

Common-Emitter Amplifiers

• Common-emitter (CE) amplifiers have the emitter as the common terminal, or ground, to an ac signal.
• CE amplifiers exhibit high voltage gain and high current gain.
• The dc equivalent circuit is used to analyze the amplifier's dc bias values, while the ac equivalent circuit is used to analyze the ac signal operation.

AC Analysis and Voltage Gain

• The ac analysis involves replacing capacitors with effective shorts and dc sources with ac grounds.
• The input resistance looking in at the base is calculated using the simplified r-parameter model of the transistor.
• The voltage gain expression for the common-emitter amplifier is developed using the model circuit.

Effect of Emitter Bypass Capacitor on Voltage Gain

• The emitter bypass capacitor provides an effective short to the ac signal around the emitter resistor, keeping the emitter at ac ground.
• The gain of the amplifier is maximum with the bypass capacitor, and the capacitive reactance should be at least 10 times smaller than the emitter resistance.

Example and Stability of Voltage Gain

• A minimum value for the emitter bypass capacitor can be selected based on the operating frequency range of the amplifier.
• The voltage gain of the amplifier can be calculated with and without the bypass capacitor, and with or without a load resistor.
• Swamping re can be used to stabilize the voltage gain, but it reduces the gain.
• The input resistance of the amplifier can be affected by swamping re.