What is the effect of a voltage series negative feedback amplifier in the following performance measures of a BJT amplifier. i) Input resistance ii) Output resistance iii) Bandwidth iv) Distortion and noise v) Gain Stability

Q.) What is the effect of a voltage series negative feedback amplifier in the following performance measures of a BJT amplifier. i) Input resistance ii) Output resistance iii) Bandwidth iv) Distortion and noise v) Gain Stability

 Subject: Electronic Devices and Circuit

i) Input Resistance:

  • Without Feedback: The input resistance of a BJT amplifier is typically high due to the high input impedance of the BJT.
  • With Feedback: Negative feedback reduces the overall input impedance of the amplifier. The amount of reduction depends on the feedback factor (β). Higher β leads to lower input resistance.

ii) Output Resistance:

  • Without Feedback: The output resistance of a BJT amplifier is usually high due to the finite resistance of the BJT's collector-emitter path (r_ce).
  • With Feedback: Negative feedback reduces the output resistance of the amplifier by providing a virtual ground at the output node. The effective output resistance is now determined primarily by the feedback network.

iii) Bandwidth:

  • Without Feedback: The bandwidth of a BJT amplifier is limited by the frequency response of the BJT and the load capacitance.
  • With Feedback: Negative feedback can extend the bandwidth of the amplifier by reducing the gain at higher frequencies. This helps to compensate for the loss in gain due to the reduction in input resistance.

iv) Distortion and Noise:

  • Without Feedback: Distortion and noise in a BJT amplifier are caused by the nonlinear characteristics of the BJT and the thermal noise generated in the BJT and other components.
  • With Feedback: Negative feedback reduces distortion and noise by linearizing the amplifier's transfer function and reducing the gain. The reduction in gain also reduces the noise contribution of the amplifier.

v) Gain Stability:

  • Without Feedback: The gain of a BJT amplifier can be affected by temperature variations, aging, and component tolerances. This can lead to gain instability and variations in the amplifier's performance.
  • With Feedback: Negative feedback stabilizes the gain of the amplifier. The feedback loop helps to maintain the desired gain by adjusting the input signal to compensate for any changes in the amplifier's characteristics.