BJT as an amplifier

Question 1

What are the components of any voltage/current in the BJT? (Notation)

Answer 1

Voltage/current values will have DC and sine wave components.

This can be represented as follows:

v_BE = V_BE + v_be

Question 2

How can we derive i_c as a function of the DC collector current, for a CE amplifier?

Answer 2

Question 3

What assumption is made in the small signal approximation (CE)?

Answer 3

v_be << V_T

This allows us to use the Taylor expansion to express the collector current as the sum of it’s DC and sine components.

Question 4

What is the equation for transconductance?

Answer 4

g_m = I_C/V_T

Transconductance is the DC collector current over the thermal voltage which is approx 25mV at room temperature

Question 5

How does transconductance relate to the collector current?

Answer 5

i_c = g_mv_be

The sinusoidal collector current is equal to the product of the transconductance and sinusoidal base-emitter voltage.

Question 6

How can we derive an expression for the sinusoidal base current (CE)?

Answer 6

We can derive:

i_c = (I_C/V_T)v_be using the small signal approximation

The base current is equal to the ratio of the collector current over beta.

Question 7

What is r_e?

Answer 7

The emitter resistance

Question 8

What is r_π?

Answer 8

The base input resistance