Exercise 2: PNP Transistor Current Control Circuit · Transistor Junctions and PNP DC Bias ... Exercise 2: PNP Transistor Current Control Circuit ... You can use a bipolar transistor

Transistor Junctions and PNP DC Bias

© Festo Didactic 91564-P0 217

Exercise 2: PNP Transistor Current Control Circuit

EXERCISE OBJECTIVE

When you have completed this exercise, you will be able to demonstrate transistor current control by

using a PNP transistor circuit. You will verify your results with a multimeter.

DISCUSSION

You can use a bipolar transistor as a switch by changing the base current (IB) from zero to its maximum

value.

The resistance between the collector (C) and emitter (E) depends on the forward current between the

base (B) and emitter (E).

When the base-emitter junction is forward biased, a base current (IB) causes the collector-emitter

resistance to become very low.


218 © Festo Didactic 91564-P0

a. reverse biased.

b. forward biased.

be about 0.5 Vdc to 0.8 Vdc more negative than the emitter.



The base current (IB) causes the collector-emitter resistance to become very low and, like a closed switch,

allows current (IC

The base-emitter junction forward bias voltage drop of a silicon transistor is about

a. 1.2 Vdc to 2.0 Vdc

b. 0.5 Vdc to 0.8 Vdc.

When the forward bias is removed by the PNP transistor base being more positive than the emitter

(reverse biased), the collector-emitter resistance becomes very high, like an open switch.



When the base-emitter junction is reverse biased, the base current (IB) becomes zero, and no current

A transistor behaves like an open switch when the

a. base-emitter junction is reverse biased.

b. base-collector junction is reverse biased.

A transistor can vary the collector current (IC) through R2 between zero (open switch) and maximum

(closed switch).

This action is caused by the base current (IB) varying between zero and the value that causes the

maximum collector current (IC).

To alter the base current (IB), change the value of the base resistor (R1). To make IB zero, reverse bias

the base-emitter junction.



The transistor collector current (IC) is controlled by the

a. voltage of the power supply.

b. transistor base current (IB).

c. type (PNP or NPN) of transistor.

When maximum collector current (IC

At saturation conditions, the collector-emitter voltage drop (VCE) approaches 0 Vdc; about all of the supply

voltage (VA) is dropped across collector resistor R2.



When the transistor base-emitter junction is reverse biased, zero collector current (ICtransistor is said to be cut off.

When the transistor is cut off, the collector-emitter voltage (VCE) equals the supply voltage (VA).

When maximum collector current (IC

a. cutoff point.

b. saturation point.

The transistor emitter current (IE) equals the sum of the base current (IB) and the collector current (IC).

IE = IB + IC

IB is very small compared to IC; as a result, the collector and emitter currents are considered almost equal.



This relationship between IC and IB is called current gain

Current Gain = IC B

The transistor collector and emitter currents

a. have a difference equal to the base current.

b. can be considered almost equal because the base current is very small.

c. vary directly with the base current.

d. All of the above

PROCEDURE

Locate the PNP DC BIAS circuit block, and connect the circuit shown.

In the circuit you connected, the PNP transistor (Q1) is

a. forward biased.

b. reverse biased.

The LED (DS1) is off because current is

a.

b.



Connect your voltmeter lead to the negative terminal of the voltage supply (VA) and the

common lead to the positive VA terminal.

Measure VA in reference to the Q1 emitter (VA positive terminal).

VA = Vdc (Recall Value 1)

Measure the voltage drop across the Q1 base-emitter junction, with reference to the emitter.

NOTE: The answer should be given in mVdc.

VBE = mVdc (Recall Value 2)

Is transistor Q1 conducting?

a. yes

b. no



Measure the voltage drop between the collector and emitter of Q1.

VCE = Vdc (Recall Value 3)

Connect the circuit shown by moving the R1 two-post connector from the plus side of the

voltage supply to the minus side.

Is transistor Q1conducting?

a. yes

b. no

Measure the voltage drop across the Q1 base-emitter junction with the meter common lead

connected to the emitter.

VBE = Vdc (Recall Value 4)

The base-emitter junction is

a. forward biased.

b. reverse biased.



With Q1 conducting, measure the voltage drop between the emitter and collector.


Transistor Q1 is operating at the

a. cutoff point.

b. saturation point.

Connect the voltmeter lead to the Q1 collector, and connect the common lead between R2

and DS1.

Measure the voltage drop across collector resistor R2.

VR2 = Vdc (Recall Value 6)



Calculate the collector current (IC) in milliamps by using the following equation based on

Ohm’s law.

IC = VR2

IC = (Step 13, Recall Value 6

IC = mA (Recall Value 7)

You can reduce collector current by decreasing the base current (making the base resistor

larger).

Place CM switch 9 in the ON position to reduce the base current by increasing the value of R1

from 10 k to 1 M .

Connect the voltmeter lead to the Q1 collector, and connect the common lead between R2

and DS1.

Measure the voltage drop across collector resistor R2.

VR2 = Vdc (Recall Value 8)

Calculate the collector current (IC) in milliamps.

IC = VR2

IC = (Step 16, Recall Value 8

IC = mA (Recall Value 9)

The decrease in IC from mA (Step 14, Recall Value 7) to

mA (Step 17, Recall Value 9) caused the LED to become

a. dimmer.

b. brighter.



Measure the voltage drop across the Q1 base-emitter junction with the meter common lead

connected to the emitter.

VBE = Vdc (Recall Value 10)

E

(PNP)

Q1B

C

R2

1k

R1

1M

DS1

VA

Was the decrease in VBE from Vdc (Step 9, Recall Value 4) to Vdc (Step

19, Recall Value 10) caused by the lower base current?

a. yes

b. no

Is the PNP transistor (Q1) base-emitter junction still forward biased?

a. yes

b. no

With Q1 conducting less, measure the voltage drop between the collector and emitter, with

reference to the emitter.


Make sure all CMs are cleared (turned off) before proceeding to the next section.



CONCLUSION

• A transistor can function like a switch by conducting (closed switch) or by not conducting (open

switch).

• The base-emitter junction has to be forward biased for the transistor to conduct; when reverse biased,

there is zero current.

• The forward bias voltage drop of the base-emitter junction for a silicon transistor is about 0.5 V to 0.8

V.

• A small base current controls a large collector current.

• The emitter current is the sum of the base and collector currents; the base current is very small

compared to the collector current.

• At the saturation point, the collector current is maximum; at the cutoff point, the collector current is

zero.

REVIEW QUESTIONS

1. Connect the circuit shown.

Place CM switch 10 in the ON position to activate it. The LED is not glowing, which indicates that no

tests show that the

a. base-emitter junction is shorted.

b. base-emitter junction is open.

c. base-collector junction is open.

d. base-collector junction is shorted.

2. When a transistor is operated as a switch, it operates

a. either at the saturation point or cutoff point.

b. in between the saturation and cutoff points.

c. always at the saturation point.

d. always at the cutoff point.



3. For a transistor to conduct, the

a. base-emitter junction has to be reverse biased.

b. base-collector junction has to be forward biased.

c. base-emitter junction has to be forward biased.

d. base resistor cannot be greater than 10 k .

4. When a transistor is not conducting, the voltage drop across the collector and emitter (VCE) terminals

a. approaches 0 Vdc.

b. is usually between 1 Vdc and 2 Vdc.

c. is not predictable.

d. is about equal to the supply voltage (VA).

5. The transistor collector current varies directly with the

a. base current.

b. type (NPN or PNP) of transistor.

c. value of the collector resistor.

d. reverse bias voltage drop of the base-emitter junction.

NOTE: Make sure all CMs are cleared (turned off) before proceeding to the next section.

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Exercise 2: PNP Transistor Current Control Circuit · Transistor Junctions and PNP DC Bias ... Exercise 2: PNP Transistor Current Control Circuit ... You can use a bipolar transistor