Student Manual

150 FACET by Lab-Volt

Bias Stabilization

When you have completed this exercise, you will be able to describe the effect of temperature on a

calculations.

An increase in transistor temperature increases beta ( , the current gain) and collector leakage current

(ICBO) and decreases the base-emitter voltage difference.

A change in beta (

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Bias Stabilization

If there is a large temperature increase, the collector current can reach the saturation point or a thermal

runaway condition that could destroy the transistor.

An increase in transistor temperature increases

a. beta ( ).

b. collector leakage current (ICBO).

c. Both of the above.

d. None of the above.

The collector leakage current (ICBO) is caused by the reverse bias voltage. ICBO increases with

temperature.

ICBO is measured from the base to the collector with the emitter open.

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Bias Stabilization

The collector leakage current (ICBO) is in the range of nanoamperes (nA), but it doubles with every 10°

Celsius increase.

The collector leakage current (ICBO) is 10 nA at 30º Celsius. At 40º Celsius, ICBO would be about

a. 10 nA.

b. 20 nA.

c. 30 nA.

The stability factor (S), a measure of transistor temperature stability, is usually measured as the ratio of

the change in collector current to the change in collector leakage current.

C

CBO

the change in IS

the change in I=

The stability factor (S) of a transistor circuit can range from a value as high as beta (20 to 500) to a value

as low as 1.0.

The lower the S value, the more stable the transistor is against temperature change.

An S value less than 10 is considered good.

A transistor is more stable against temperature change when the transistor stability factor is

a. low.

b. high.

).

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Bias Stabilization

BE, which causes an increase in both

the voltage drop across R3 and the base current.

The base current increase causes an increase in collector current.

The increase in beta and in collector leakage current compounds the collector current increase with base

current.

Student Manual

154 FACET by Lab-Volt

Bias Stabilization

a. decrease in VBE.

b. increase in the voltage drop across R3.

c. increase in the base current.

d. All of the above.

at cutoff (open) or saturation (closed).

a. switches.

b.

c.

Locate the BIAS STABILIZATION circuit block.

Turn potentiometer R3 fully counterclockwise (CCW) for maximum resistance.

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Bias Stabilization

Adjust the positive variable dc power supply so that VA equals 6.0 Vdc.

Adjust potentiometer R3 for 0.20 Vdc across R5 in the collector circuit.

If R3 cannot be adjusted for 0.20 Vdc across R5, disconnect R1 and connect R2 to the circuit.

Adjusting potentiometer R3 for 0.20 Vdc across R5 sets the collector current (IC).

Calculate the collector current.

IC = mA (Recall Value 1)

Student Manual

156 FACET by Lab-Volt

Bias Stabilization

You will observe the change in base-emitter voltage (VBE) and collector current (IC) due to an

increase in transistor Q1 temperature after the transistor is heated for 2 minutes.

On your circuit board, the HEATER is the resistor that is physically on top of transistor Q1.

Measure the base-emitter voltage (VBE(cold)).

VBE(cold) = Vdc (Recall Value 2)

Connect the transistor HEATER to the circuit by using a two-post connector, and make note

of the time.

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After the HEATER is connected for 2 minutes, measure VBE(hot).

VBE(hot) = Vdc (Recall Value 3)

Disconnect the HEATER from the circuit.

Does VBE decrease or increase when the transistor Q1 temperature increases?

a. increase

b. decrease

VBE(cold) = Vdc (Step 7, Recall Value 2)

VBE(hot) = Vdc (Step 8, Recall Value 3)

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Bias Stabilization

What is the change in VBE after the transistor is heated for 2 minutes?

The change in VBE = Vdc (Recall Value 4)

Wait at least 10 minutes for transistor Q1 to cool.

Then, if necessary, readjust R3 for 0.20 Vdc across R5.

Enter the voltage across R5 (VR5(cold)).

VR5(cold) = Vdc (Recall Value 5)

Calculate the collector current (IC(cold)).

IC(cold) = mA (Recall Value 6)

Connect the transistor HEATER to the circuit, and make note of the time.

After the HEATER is connected for 2 minutes, measure the voltage across R5 (VR5(hot)).

VR5(hot) = Vdc (Recall Value 7)

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Bias Stabilization

Disconnect the HEATER from the circuit.

What is the collector current (IC(hot)) after the transistor is heated for 2 minutes?

VR5(hot) = Vdc (Step 15, Recall Value 7)

IC(hot) = mA (Recall Value 8)

Does IC decrease or increase when the transistor Q1 temperature increases?

a. decrease

b. increase

IC(hot) = mA (Step 17, Recall Value 8)

IC(cold) = mA (Step 14, Recall Value 6)

What is the change in IC after the transistor is heated for 2 minutes?

the change in IC = mA (Recall Value 9)

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Bias Stabilization

What is the percentage (%) of change in collector current (IC) after transistor Q1 is heated

C(hot) C(cold)

C(cold)

I I100

I

−

×

% change = percent (Recall Value 10)

• Transistors are temperature sensitive.

• A change in beta (

• The base-emitter voltage (VBE) decreases with a transistor temperature increase.

•

with temperature change.

•

point (on) or the cutoff point (off).

1. Transistors are

a. not very sensitive to changes in temperature.

b. heat sensitive.

c. reliable when operated above 75º Celsius.

d. pressure sensitive.

2. Transistor bias refers to the

a. dc operating conditions.

b. ac operating conditions.

c. temperature stability.

d. dc voltage supply.

3. The transistor base-emitter voltage (VBE)

a. increases with an increase in temperature.

b. is not affected by temperature change.

c. decreases with an increase in temperature.

d. has no effect on collector current.

4.

a. toward the cutoff point.

b.

c. to a new dc load line.

d. toward the saturation point.

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Bias Stabilization

5. A transistor circuit with a stability factor of 200

a. is not affected by a temperature change.

b. has very poor temperature stability.

c.

d. is usually not affected by a change in beta ( ) due to a temperature change.