BJT - Bias circuits

Question 1

What is the circuit layout of a base bias circuit?

Answer 1

Question 2

What is the advantage of a constant base bias?

Answer 2

The base current is well defined since V_BE stays substantially constant (0.7), this is due to i_B being fixed.

Question 3

What is the disadvantage of constant base bias?

Answer 3

The value of R_B is heavily dependent on ß, which is poorly specified for most transistors.

ß can vary between 30 and 300 for a given device.

Question 4

What is the circuit layout for shunt feedback biasing?

Answer 4

Question 5

What is the circuit diagram for potentiometer biasing?

Answer 5

Question 6

What is the rule of thirds?

Answer 6

If we let the base voltage (or emitter voltage) = 1/3 of the supply voltage, V_cc

then we can assume,

V_CE = 1/3 V_CC, the collector-emitter voltage is one third of the voltage supply.

and I_cR_c = 1/3V_cc, the collector resistor voltage is one third of the supply voltage

Question 7

If we are not using the rule of thirds what other assumption can we make to solve a bias circuit?

Answer 7

We can assume that the voltage across V_CE (collector-emitter voltage) is half of the supply voltage, V_CC.

Question 8

If we have a potentiometer biased circuit, a voltage drop across V_E that does not follow the rule of thirds, what assumptions can we make?

Answer 8

We can usually assume that I_C = I_B unless specified otherwise.

We can also use the V_CE = V_CC/2, however we must account for the voltage drop across V_E.

This would mean that,

V_C = (V_CC - V_E)/2

and

V_CE = (V_CC - V_E)/2 + V_E

Question 9

What is the circuit diagram for the Thevenin equivalent of the potentiometer biasing circuit?

Answer 9

Question 10

Describe the potentiometer biasing circuit

Answer 10

A single power supply is used.

The technique consists of supplying the base of the transistor with a fraction of supply voltage VCC through the voltage divider R1 and R2.

In addition, a resistor RE is connected to the emitter and a resistor RC is connected between VCC and the collector at a point VC.

Question 11

Describe the Thevenin equivalent of the potentiometer biasing circuit.

Answer 11

The voltage divider which includes R1 and R2 is replaced by a single resistor denoted by RB which is the parallel combination of R1 and R2.

The value of the thevenin equivalent voltage source will be:

Question 12

What is the equation for emitter current for the Thevenin equivalent circuit?

Answer 12

Question 13

What is the equation for collector voltage in a potentiometer bias circuit?

Answer 13

Question 14

How can we design a bias circuit to ensure the emitter current is insensitive to temperatue and ß variation?

Answer 14

We must satisfy the following constraints