Test: Feedback Amplifiers - Electronics and Communication Engineering (ECE) MCQ

To find the closed loop voltage gain of the amplifier circuit, follow these steps:

• The open loop gain of the circuit is given as 40.
• Assume an ideal current source and neglect the effects like body effect, channel length modulation, and intrinsic capacitances.
• All transistors are biased in saturation, ensuring they operate in their intended region.
• The closed loop voltage gain v_out / v_in is calculated using the formula for feedback amplifiers:

The closed loop gain is approximately 0.976 after calculations.

To determine when a cascade of common-source amplifiers in a unity gain feedback configuration oscillates, we need to consider both the gain and the phase shift conditions of the system. The critical criteria for oscillation can be derived from the Barkhausen stability criterion, which states:

1. The loop gain (product of the amplifier gain and the feedback factor) must be equal to or greater than 1.

2. The total phase shift around the loop must be an integer multiple of 360^∘ (or 0^∘ modulo 360^∘).

In the context of unity gain feedback configuration:

If the closed-loop gain is greater than 1, this means the amplitude condition for oscillation is satisfied.

For the phase condition, if the phase shift is greater than 180^∘, it ensures that additional circuitry (or inherent amplifier phase shift) can bring the total phase shift around the loop to precisely 360^∘ or 0^∘ modulo 360^∘, thus satisfying the phase condition for oscillation.

Hence, a cascade of common-source amplifiers in a unity gain feedback configuration will oscillate when:

Option D: the closed loop gain is greater than 1 and the phase shift is greater than 180^∘.

The type of feedback in the given circuit is voltage-current feedback. Here's why:

• The feedback involves both voltage sensing and current sampling.
• This means the output voltage is sampled and fed back into the input as a current, hence the name voltage-current feedback.
• Such feedback is useful for stabilising the system and controlling the gain.

A current buffer is a device that helps maintain the stability of an electrical circuit. For optimal performance, it should have:

• High input impedance: This ensures minimal current is drawn from the source, preventing signal distortion.
• Low output impedance: This allows the buffer to drive loads effectively, maintaining the signal's integrity.

These features help in isolating different stages of a circuit, ensuring that the interaction between them is minimised.

The desirable characteristics of a transconductance amplifier include:

• High input resistance ensures minimal loading on the signal source.
• High output resistance allows for effective current transfer to the load.

The amplifier circuit uses a current series feedback topology. This type of feedback is characterised by:

• Improving the stability of the amplifier by reducing the gain variability.
• Increasing the bandwidth, allowing the circuit to handle a wider range of frequencies.
• Enhancing the linearity of the amplifier, which improves the signal quality by reducing distortion.

When the gaink is increased in a voltage-voltage feedback circuit:

• The input impedance increases.
• The output impedance decreases.

In a shunt-shunt negative feedback amplifier, the effects compared to a basic amplifier are:

• Both input and output impedance decrease.
• The feedback reduces the amplifier's gain.
• This configuration improves stability and reduces distortion.

To achieve high input and high output impedances in a feedback amplifier, the most commonly used configuration is:

• Current-series feedback is often preferred for this purpose.
• This configuration effectively increases the input impedance.
• It also increases the output impedance, making it suitable for high impedance requirements.

​​​​​​​Negative feedback in amplifiers can have several effects:

• It improves the signal-to-noise ratio at the input, making the signal clearer.

• It improves the signal-to-noise ratio at the output as well.

• It reduces distortion, resulting in a more accurate reproduction of the input signal.