ECE121 Experiment No. 4 - Unijunction Transistor

UNIJUNCTION TRANSISTOR

Laboratory Exercise No. 4

Performance Objectives

A. Measure the interbase resistance and determine the emitter-base 1 PN junction diode characteristics of a unijunction transistor.

B. Determine the intrinsic standoff ratio of a unijunction transistor.

C. Measure the peak emitter firing voltage of a unijunction transistor.

Equipment and Materials

ü Power Source 0-10 Vdc, 20mA

ü Electronic VOM

ü Bread board

ü C1 0.22μF

ü C2 0.1μF

ü CR1 Silicon Diode, IN4004

ü Q1 Unijunction Transistor, 2N2646

ü R1 10KΩ, 1W

ü R2 100KΩ, 1W

ü R3 1KΩ, 1W

ü R4 10KΩ, 1W

Objective A. Measure the interbase resistance and determine the emitter-base 1 PN junction diode characteristics of a unijunction transistor.

1. a) Look at the UJT and identify the base 1, base 2, and emitter leads. Set the Electronic VOM to the ohmmeter function on the R x 1K range.

b) Measure and record the resistance between B1 and B2 with the emitter open (R_BBO).

R_BBO = 5.45 KΩ

c) Reverse the ohmmeter leads and measure R_BBO again.

R_BBO = 5.38 KΩ

d) Do your measurements of R_BBO indicate that the UJT has a PN junction between B1 and B2? No.

e) Measure and record the forward emitter to base 1 resistance. Connect the ohms probe of the ohmmeter to the emitter and the ground lead to base 1.

R_{EB1 (forward)} = ___KΩ

f) Reverse the ohmmeter leads and measure the reverse resistance between emitter and base 1.

R_{EB1 (reverse)} = ∞

g) Compare your R_EB1 measurements. Do these values indicate the UJT has a PN junction between emitter and base 1? Yes.

Objective B. Determine the intrinsic standoff ratio of a unijunction transistor.

2. a) Examine the circuit shown in Fig. 5-2. Capacitor C2 changes through R1 until it reaches the firing voltage of the UJT. When Q1 fires and Q2 discharges through the emitter-base 1 circuit of the UJT, the charge on C2 then drops below the value required to hold Q1 in conduction and Q1 cuts off again.

Figure 5-2

b) Connect the circuit shown in Fig. 5-2.

c) Adjust V_BB to 10 Vdc.

d) Measure and record the peak voltage across C1.

E_C1 = 0.679 Volts

e) Calculate the intrinsic standoff ratio using the V_BB value of (c) and E_C1 of (d).

Ŋ = Vp/ V_BB = E_C1/V_BB

Ŋ = 0.679/10

­ Ŋ = 67.9 mV

f) Reduce V_BB to zero.

Objective C. Measure the peak emitter firing voltage of a unijunction transistor.

3. a) Now change your circuit to that shown in Fig. 5-3. Adjust R4 for minimum resistance initially.

Figure 5-3

b) Adjust V_BB to 10 Vdc.

c) Slowly increase the resistance of R4 while monitoring the voltage from emitter to ground on the Electronic VOM. What happens to the voltmeter reading?

As the resistance is increased, the voltage across UJT also increases until it reaches the maximum voltmeter reading. It attains the highest voltage then drops to a small value.

d) What does the peak reading on the voltmeter represent?

The voltage across the voltmeter represent the voltage to trigger your UJT to “on”.

e) What causes the sudden decrease in emitter voltage?

The decrease in emitter voltage is a result that the UJT was triggered by a positive voltage. When the UJT is on, the resistance across the base decreases.

f) Reduce R4 to minimum resistance again.

g) Repeat (c) and (f) as many times as necessary until you obtain an accurate measure of Vp.

Vp = 7.06 volts

h) Compare the value of Vp with the value E_C1 of in 2(d). Do the values agree? Explain any discrepancy.

No, they do not agree. They have small difference and the resistance with respect to ground differs at least 1KΩ.

i) What effect would a larger value of _ Vp?

The effect of a larger Vp will increase the voltage in the emitter.

j) What effect would a larger value of V_BB have on Vp?

If V_BB would have a large value, Vp will also increase because diode voltage and h are fixed values. Thus, varying V_BB will affect Vp. (Vp = V_D + h V_BB)

k) Reduct the power supply voltage to zero.

Conclusion

• Since there is an approximately equal resistance between the bases of the unijunction transistor, it can be concluded that there is they have no polarity, thus has no PN junction. While the emitter-base resistance has polarity so there is PN junction between them – positive on the emitter and negative on either base. R_BB = R_B1 + R_B2 when I_E= 0.
• Based on the experiment, the intrinsic standoff ratio of a unijunction transistor is defined by Ŋ = Vp/ V_BB = E_C1/V_BB where Vp has a small voltage value because of a small resistance in base 1.

• By increasing the resistance with respect to emitter also increases the voltage entering the UJT therefore triggering it to turn ‘on’. When V_E = Vp, conduction is established and emitter potential V_E will drop with increase in I_E. A larger Vp will increase the voltage in the emitter.