Understanding Negative Feedback in Series Voltage Amplifiers

Negative feedback is a technique widely used in amplifiers to improve their performance characteristics. In a series voltage negative feedback amplifier, a portion of the output voltage is fed back in series with the input voltage, in a subtractive manner. This feedback loop significantly alters the amplifier's properties.

Effects of Negative Feedback

Let's examine how negative feedback affects the key characteristics of a series voltage negative feedback amplifier:

• Voltage Gain: Negative feedback reduces the overall voltage gain of the amplifier. The closed-loop voltage gain ($A_{vf}$) is related to the open-loop voltage gain ($A_v$) and the feedback factor ($\beta$) by the formula: $$ A_{vf} = \frac{A_v}{1 + A_v\beta} $$ As you can see, the gain is reduced by the factor $(1 + A_v\beta)$.
• Input Impedance: For a series feedback configuration at the input, negative feedback increases the input impedance. The new input impedance ($R_{in,f}$) is related to the original input impedance ($R_{in}$) by: $$ R_{in,f} = R_{in} (1 + A_v\beta) $$ So, input impedance increases by a factor of $(1 + A_v\beta)$.
• Output Impedance: For a voltage feedback configuration at the output, negative feedback decreases the output impedance. The new output impedance ($R_{out,f}$) is related to the original output impedance ($R_{out}$) by: $$ R_{out,f} = \frac{R_{out}}{1 + A_v\beta} $$ The output impedance is reduced by the factor $(1 + A_v\beta)$.
• Bandwidth: One of the significant benefits of negative feedback is the increase in the amplifier's bandwidth. The bandwidth ($BW_f$) of the amplifier with negative feedback is related to its original bandwidth ($BW$) by: $$ BW_f = BW (1 + A_v\beta) $$ This shows that the bandwidth increases by a factor of $(1 + A_v\beta)$.

Analyzing the Question

The question asks what characteristic of a series voltage negative feedback amplifier is increased by a factor of $(1 + A_v\beta)$. Based on our analysis of the effects of negative feedback:

The question specifically states the characteristic is increased by a factor of $(1 + A_v\beta)$. Both input impedance (for series feedback) and bandwidth increase by this factor. However, among the given options, 'bandwidth' is listed. Therefore, the characteristic that is increased by a factor of $(1 + A_v\beta)$ in a series voltage negative feedback amplifier is the bandwidth.

Negative feedback extends the frequency range over which the amplifier provides useful gain, effectively increasing its bandwidth. This trade-off of reduced gain for increased bandwidth, along with other performance improvements, makes negative feedback a valuable technique in amplifier design.