Lab 3: Common Emitter Amplifier

OBJECTIVES

Experience in design of a single stage amplifier. Simulation of circuit performance with Multisim and comparison with measured values.

DESIGN PRINCIPLES

To work properly in an amplifier circuit a bipolar junction transistor (BJT) must be properly biased and operate in the active mode. This is achieved by a proper resistor network. The following conditions should be established:

1. The collector should be biased half way between ground and V_CC so that the output can swing both way equally. This defines the value of R_C for a given collector quiescent current (with no input signal).
   
   
2. The emitter should be biased at about 1 V to achieve temperature stability. This defines the value of R_E (assume that emitter and collector quiescent currents are the same). Temperature stability comes from a negative feedback: If the transistor heats up and the emitter current increases, the voltage drop across R_E increases, decreasing the voltage difference between the base and the emitter, thus decreasing the current through the transistor and lowering its temperature.
   
   
3. Find the ratio of R₁:R₂ to put the base ~0.6V above the emitter (at 1.6 V).
   
   
4. Chose R₃ for the required gain G = R_C/(R_E || R₃'), where R₃' = R₃+ r_e, with r_e ~ 50 Ω being the "intrinsic emitter resistance".
   
   
5. Select the values of C₁ and C₂ for the required f-3dB. Note that C₁ is a component of an R_C filter with R_in = Rb || β (R₃+r_e). R_b is the equivalent resistance of the base biasing network (R₁ || R₂) and β ~ 100.

PRELAB

Design a common emitter amplifier following the schematic shown in the figure above (for more information see ref. 1)

Design requirements:

1. Gain at quiescent point between 40 and 100, specified by the instructor for each group;
   
2. f_-3dB must be not larger than 200 Hz;
   
3. V_cc = 15 volts;
   
4. Quiescent current I_c = 0.5 mA.

Specify component values to meet these requirements but check also the parts selection in your kit. Obtain components from other sources, if needed. You may deviate from the design specification by 10% if you do not find the required components.

Check your design by simulating circuit performance with Multisim using the same signals as you will use in the laboratory.

LABORATORY

1.      ASSEMBLY AND TESTING

Wire the amplifier circuit following your design. Measure V_E, V_B, and V_C in quiescent conditions. Drive the input with small sinusoidal signal (output should not exceed 1 V) and measure the values of the design parameters: small signal voltage gain, quiescent current, and the frequency f_-3dB. Use DVM for accuracy but note the phase relation between input and output waveforms on the oscilloscope.

Measure also input and output impedances (R_in and R_out). To find input impedance, connect a series resistor to the input and measure voltage across it. The resistor should be large enough to measure the ac voltage across it. You may use the scope probes on the two ends of the resistor and measure the voltage displaying the difference between the two channels. To find output impedance, load the output with a resistor R_L (through a capacitor – you need ac only!) and measure the output voltage drop due to the load. The voltage divider formula (with unknown R_out and known R_L) will give you R_out.

NOTE: Circuits with high gain may oscillate. To prevent this, make neat and short connections between components and pay attention to grounding. Fixed resistors and capacitors are preferred to resistance and capacitance boxes.

2.      FINE TUNING

If the measurements do not agree with the design parameters, fine tune your circuit by replacing or adding resistors. You may also introduce a trimming (adjustable) resistor for convenient tuning. Do not be concerned with obtaining an exact value of f_-3dB as long as it is not larger than specified.

REPORT

In the report compare the design parameters with their measured values. Describe any fine tuning which had to be done on the circuit. Compare measurements with simulation results from Multisim, including the measured f_-3dB vs. the value from simulated frequency distribution of the circuit. Compare measured and calculated values of input and output impedances.

To improve this amplifier you could expand this circuit by adding more stages (transistors).

What would you do to: (a) Increase the input impedance (b) Decrease the output impedance? Hint: recall previous experiments.