EXPERIMENT NO. 21: THE LOAD LINE AND Q-POINT

OBJECTIVE

1. To learn to use the characteristic curves for graphical analysis of an amplifier.
2. To project the operation characteristics of an amplifier for single state DC and AC operation.
3. To observe graphically how input signal variations produce output signal variations.

EQUIPMENT

Use the circuit model " EXP 21".

INTRODUCTION

A knowledge of the charateristic of a particular transistor enables the designer to predict the performance of an amplifier circuit using the trasistor.Although graphical analysis not commonly required for small-signal amplifiers, a study of the concepts involved can be very useful.

Applying Kirchoff's voltage law to the output of the simple amplifier circuit in figure 1 gives

V_CC = I_C + V_CE

or solving for I_C

I_C= - V_CE/R_L + V / R_L

This expression represents a straight line which is known as the load line and is shown in figure 2 superimposed on the assumed transistor ouput characteristics. The load line is the locus for all voltages and currents that can exist with the given load connected in the given transitor circuit .In practice , the load line is easily drawn using the intercept on the horizontal axis (=Vcc) and the vertical axis (=V_CC/R_L).

The actual values od collector current (I_C) and collector-emitter voltage(V_CE) define the operating point or quiescent point ( Q point) of the circuit and must be on the load line .In practice V_CE is arbitarily chosen (commonly with a value around V_CC/2) while R_L can be selscted to provide the desired value of Ic.

In figure 2 :

• V_CC = + 20V ( intercept on the horizontal axis )
• V_CC/R_L= +40V mA (intercept on vertical axis)
• RL = 0.5K , or 500 ohm.

  While the Qpoint is at

• V_CE = + 12V , I_C = + 16mA ,I_B = 200µA

If a sinusiodal signal is superimposed on the input circuit ,the base current Ib will vary in the manner indicate in figure 2 ,where it is assumed that the signal has a peak value of 100µA (200µA peak-peak). The result is that the quiescent value of Ib is veried by ±100µA , that is , it oscillates between 100 and 300µ as shown.

As the input signal moves positive , the operating point moves along the load line up to A at the positive peak ,back to Q and down to B at the negative peak . The output current Ic veries correspondingly as shown in the diagram. The collector-emitter voltage also veries with change in Ib but it is important to note that there is a phase reversal so that Vce decrease as Ib increases and Vce increases as Ib decreases.

PROCEDURE

1. Connect the circuit as shown in Figure 3 . The Mini-Lab bipolar power supply can be used for the base supply. The 33 ohm resistor and the diodes are provided for protection purposes only.
2. Adjust the base current to 5 µA and measuse the collector voltage and current for collector voltages of 0.2,0.4,0.6,0.8,1,2,4,6,8 and 10V.

   Repeat for base currents of 0,10,15,20,30,40 and 50 µA.

3. Connect the circuit of Figure 1 with V_CC = +10V, R_L = 2.2K and measure I_C and I_B with Ib adjusted to give Vce =+5V.

REPORT

1. Plot the transitor output characteristic on graph paper.
2. On the same graph ,draw a load line for V_CC = +10V , R_L = 2.2K and find the quiescent point for V_CE = +5V. Compare the values of I_B and I_B and I_C for this Q-Point with those found experimentally.

   What is the value of DC gain (ß_DC or h_FE for the transitor at this point?

3. Using the same load line and Q point ,select suitable points A and B on the load line corresponding to an equal increase and decrease respectively, in base current - similar to that shown in Figure 2.

4. Using points A and B ,find the predicted peak-to-peak values of I_B, I_C and V_CE .What is the value of an Ac gain (ßac or h_FE) for the transitor at this Q poin?

5. Briefly discuss the concept of a 'small signal'.

For assistance in connecting up your circuit for this experiment, click on bar above.