Amplifier Teaching Aid (DED Philippinen, 86 p.) Lesson 4 - Transistor Fundamentals Lesson Plan (introduction...) The load line The operating point Recognizing saturation The transistor switch Worksheet No. 4

Amplifier Teaching Aid (DED Philippinen, 86 p.)

Lesson 4 - Transistor Fundamentals

Lesson Plan

Title: Transistor Fundamentals

Objectives:

- Understand the meaning of load line and Q-point
- Able to do the calculation for load line and Q-point
- Know the principle of a transistor switch

Figure

The load line

The load line contains every possible operating point for the circuit. A line is defined by two points. To draw the load line you have to get. the saturation point and the cutoff point:

Saturation point: Tells you the maximum possible collector current for the circuit.

Calculate: Visualize a short between the collector and emitter.

V_CE --- > 0

Cutoff point : Tells you the maximum possible collector emitter voltage for the circuit. Calculate: Visualize the transistor internally open between collector and emitter.

V_CE -- > V_CC

V_{CE (cut)} = V_CC

Ex: Draw the load line for the given circuit.

Fig. 4-1: CE amplifier base biased

Fig. 4-2: Output curve

Saturation point:

Cutoff point:

V_{CE (cut)} = V_CC = 15V

Fig. 4-3: Output curve with loadline

HO: Suppose the collector resistance (in Fig. 4-1) is increased to 6KW. What happens to the dc load line?

Solution:

V_{CE (cut)} = 15V

Figure

The operating point

Every transistor circuit has a load line. If the base resistance is given you can also calculate the current and voltage for the operating point.

Ex: Calculate the operating point (Q-point)

Fig. 4-4: Base biased CE connection

I_C = b_dc * I_B = 100 * 29mA = 2.9 mA

V_CE = V_CC - (I_C * R_C) = 15V - (2.9 mA * 3KW) = 6.3V

By plotting I_C (2.9 mA) and V_CE (6.3V), we get the operation point ----> Q-point (quiescent point).

Fig. 4 - 5: Collector curve with load line and Q - point

HO: Draw the load line and Q-point.

Fig. 4-6: base biased CE connection, b=50

Solution:

I_C = I_B * b = 2.15 mA

V_CE = V_CC - (R_C * I_C)= 5.7V

V_{CE (cut)} = V_CC = 3.0V

Figure

Recognizing saturation

When you first look at a transistor circuit, you usually cannot tell if it is saturated or operating in the active region.

1. calculate I_{C (sat)}

2. calculate I_C

If I_C is greater than I_{C (sat)} the transistor is saturated.

Note: Current gain is less in saturation region.

The transistor switch

Ex: Circuit example for a transistor switch:

Fig. 4-7: Transistor switch

The transistor operate only at saturation and cutoff

Switch closed: Transistor in hard saturation V_out » 0V

Switch open : Transistor in cutoff

I_C drops to zero

V_out » 10V

Worksheet No. 4

Figure

No. 1 Draw the load line!

No. 2 If the collector resistance is increased to 1K, what happens to the load line?

No. 3 What is the voltage between the collector and ground if the current gain is 100?