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

Concept:

V_T & I_T method:
Deactivate all the independent sources and do not disturb the dependent sources.
Rth = V_T / I_T

Deactivate source:
voltage source →  short circuit  i.e V = 0
current  source → open  circuit  i.e I = 0

Analysis:
While calculating Ro in the amplifier we apply V_T  & I_T method at the output.
Apply V_T  & I_T method to calculate R₀
R0 = V_T / I_T While deactivating sources 
R0 is calculated when V₀ is short circuit i.e  V_s = 0. 

Concept:
Voltage gain (A_v): Voltage gain (A_v) is given as 

Av in dB is given as
AvdB = 20 log(A_v)
Analysis:
AV dB = 20 log(A_v) 
We know that for x >1, log(x) is always positive 
Therefore A_v >1, log(A_v) is always positive 
For  A_v > 1
AvdB is positive

Concept:
Common mode signal:

• A common-mode signal is one that drives both inputs of a differential amplifier equally
• The common-mode signal is interference, static and other kinds of undesirable pickup etc

Application:

• The transistors in the differential amplifier are chosen to be closely matched
• If both the transistors were matched in all respects then the balanced output would be theoretically zero
• This is the important characteristic of a differential amplifier
• It discriminates against common-mode input signals i.e. it refuses to amplify the common-mode signals

• Negative feedback reduces the gain of the system by a factor of (1 + Aβ).
• Thus, because of feedback configuration, voltage gain will be decreased by a factor (1 + Aβ).
• Voltage series type feedback tells us that the input of the system is series connected with the feedback loop and series connection corresponds to increased impedance.
• Therefore the input impedance will be increased by a factor (1 + Aβ).

Multistage amplifier:

• In practical applications, the output of a single-stage amplifier is usually insufficient, though it is a voltage or power amplifier .hence they are replaced by multi-stage amplifiers.
• In multi-stage amplifiers, the output of the first stage is coupled to the input of the next stage using a coupling device.
• These coupling devices usually be a capacitor or a transformer, this process of joining two amplifier stages using a coupling device is called cascading
• Now the overall gain is equal to the product of individual gains and hence the gain increases.
• The frequency and bandwidth gets reduced.

Concept:
For the negative feedback amplifier, the closed-loop gain is given by:

For the positive feedback amplifier, the closed-loop gain is given by:

Where AOL is the open-loop gain
β is the gain of the feedback
Calculation:
The gain of feedback (β) = 10/100 = 0.1
Gain without feedback = 20
Gain with negative feedback:


⇒ 6.67
Gain with feedback = 6.67

CE amplifier with unbypassed emitter resistance:

Performance parameters:

• Lower voltage gain which provides better bias stabilization.
• It works as a current series feedback network.
• Input impedance increases.
• Provide linear (undistorted) operation for larger input signals.

An op-amp has the following characteristics:

• Input impedance (Differential or Common-mode) = very high (ideally infinity)
• Output impedance (open loop) = very low (Ideally zero)
• Voltage gain = very high (ideally infinity)
• Common-mode voltage gain = very low (ideally zero), i.e. Vout = 0 (ideally), when both the inputs are at the same voltage, i.e. (zero "offset voltage")
• Output can change instantaneously (Infinite Slew Rate)
• The purpose of bias current is to achieve the ideal behavior in op-amp which is high CMRR, high differential gain, and high input impedance

Concept:
Expected power level = Input power – Losses + Total Gain
Note: In dB, the gains are simply added.
To convert dBm to dBW and vice-versa:
P(dBm) = P(dBW) + 30
Calculation:
Given:
Input Power = -10 dBm
- 10 = P + 30
Input power = -40 dBW
Net losses = – 2 – 3 – 5 = –10 dBW
Net Gain = 20 + 30 = 50 dBW
Expected power level  will be:
= –40 – (2) + 30 – (3) + 20 – 5 = 0 dBW
P(dBm) = 0 + 30 
∴ 30 dBm will be the correct answer.

Concept:
Ideal common collector (CC) amplifier: cc amplifier is also known as 
(1) unity gain amplifier (AV≈ 1)
(2) voltage follower
(3) voltage-Buffer
(4) Impedance matching
Analysis:
In an ideal cc amplifier voltage gain is 1

V_o = V_i