Explain the working of transistor as an amplifier. What factors are considered for selecting operating point Q for an amplifier.

Working of a Transistor as an Amplifier

A transistor can be used as an amplifier by utilizing its ability to control the flow of current through its collector-emitter junction based on the current flowing through its base-emitter junction. The most common types of transistors used as amplifiers are Bipolar Junction Transistors (BJTs) and Field-Effect Transistors (FETs). Here, we will focus on the BJT, which has two types: NPN and PNP. We will use the NPN transistor for our explanation.

Step-by-Step Working of an NPN Transistor Amplifier:

1. Basic Structure: An NPN transistor has three terminals: the base (B), the collector (C), and the emitter (E). In an NPN transistor, a small base current controls a larger collector current.

2. Biasing: To operate the transistor as an amplifier, it must be properly biased. This means applying a DC voltage to the base-emitter junction to turn it on (forward-bias) and to the collector-base junction to reverse-bias it.

3. Signal Input: An AC signal, which is to be amplified, is superimposed on the DC bias at the base terminal.

4. Amplification Process: As the input signal fluctuates, it causes the base-emitter current to vary. Because the transistor has a current gain (β), this variation in base current causes a proportionally larger variation in the collector current.

5. Output Signal: The fluctuating collector current flows through a load resistor connected to the collector terminal, creating a voltage across it. This voltage is the amplified output signal.

6. Phase Relation: In an NPN transistor amplifier, the output signal is 180 degrees out of phase with the input signal.

Factors for Selecting Operating Point Q for an Amplifier

The operating point, also known as the Q-point, is the point on the transistor's characteristic curve where it operates to provide the desired amplification with minimal distortion. The following factors are considered when selecting the Q-point:

Factor	Description
Stability	The Q-point should be stable against temperature variations and transistor parameter changes.
Linearity	To minimize distortion, the Q-point should be in the linear region of the transistor's characteristic curve.
Maximum Undistorted Output	The Q-point should allow for the maximum undistorted output signal swing.
Biasing Scheme	The biasing scheme (fixed bias, voltage divider bias, etc.) affects the stability and positioning of the Q-point.
Power Consumption	The Q-point should be chosen to optimize power consumption without compromising performance.
Load Resistance	The value of the load resistor affects the gain and the position of the Q-point.

Example of Q-point Selection

Let's consider a common-emitter amplifier circuit using an NPN transistor. The DC load line is drawn on the output characteristic curves of the transistor by applying Kirchhoff's voltage law:

[ V_{CC} = I_C R_C + V_{CE} ]

Where:

• ( V_{CC} ) is the supply voltage
• ( I_C ) is the collector current
• ( R_C ) is the collector resistor
• ( V_{CE} ) is the collector-emitter voltage

The Q-point is chosen where the DC load line intersects the transistor's characteristic curve. Ideally, it is selected at the center of the load line to allow for equal signal swings in the positive and negative directions without hitting saturation or cutoff.

For example, if ( V_{CC} ) is 12V and ( R_C ) is 1kΩ, the maximum collector current ( I_C ) when ( V_{CE} ) is 0V is:

[ I_C = \frac{V_{CC}}{R_C} = \frac{12V}{1kΩ} = 12mA ]

The maximum ( V_{CE} ) when ( I_C ) is 0A is 12V. The Q-point should be chosen around ( I_C = 6mA ) and ( V_{CE} = 6V ) for maximum undistorted output.

By carefully selecting the Q-point and ensuring proper biasing, the transistor amplifier can provide a stable and linear amplification of the input signal with minimal distortion.