Electronic Circuits (Part 2)

Question 1

What sort of transistor is this?

How can you tell?

Answer 1

You can tell by the direction of the arrow.

In this case, it is pointing out and is on the emitter side, so this is an npn BJT.

The arrow also indicates that the region is more heavily doped than the other two regions. It also shows current flow(movement of positive charge)

Question 2

What is the relationship between I_C and I_B in a npn BJT?

Answer 2

I_C >> I_B

𝛽 ₌ I_C / I_B

𝛽 usually a large number between 100 and 200

Question 3

If asked to do a BJT DC analysis, what should you remember?

Answer 3

• Assume V_BE = 0.7 V and assume the BJT is in the forward-active region
• That 𝐼_𝐶 = 𝛽𝐼_𝐵
• I_C is independent of V_CE as long as the BJT is in the forward-active region
• Once all the voltages have been determined, check if V_C ≥ V_B . If not, then the BJT is not in the forward active region and the results are not valid.

Question 4

What happens when V_CE is less than about 0.5V?

V_CE refers to the voltage between the collector and the emitter.

Answer 4

When V_CE is less than about 0.5V, the base-collector junction starts to be forward biased and the BJT is in the saturation region. As a result, I_C ≠ β*I_B

Question 5

What does a BJT in the forward-active region require?

Answer 5

That the base-emitter junction is forward biased and base-collector junction is reverse biased.

Question 6

What’s another thing you should remember about solving BJT DC analysis problems ?

Answer 6

If you cannot clearly identify the voltage drops, focus on determining currents first.

Question 7

What is a coupling capacitor?

Answer 7

A coupling capacitor is used to join two circuits such that simply an AC signal from the first circuit is able to pass throughout the next circuit while the DC is blocked.

This allows the isolation of the DC bias setting of the two coupled circuits.

Question 8

What do coupling capacitors allow in a transistor amplifier circuit?

Answer 8

Capacitors resemble an open circuit at DC, but a short circuit for AC signals.

Thus, they can be used to amplify an AC signal input without disturbing the DC bias.

Question 9

For a common-emitter amplifier, what are the coordinates of the operating point?

Answer 9

Collector Current (I_C) and the voltage across collector and emitter V_CE .

Question 10

What is transistor biasing?

Answer 10

Transistor biasing is controlling the amount of current and voltage that goes into a transistor so that it produces the desired amplification of switching effect.

Question 11

“Circuit biasing not only sets the quiescent points…”

Answer 11

“…but also determines the small signal parameters.”

Question 12

Why is a capacitor open circuit for a DC power source?

Answer 12

Its a open circuit because if you look at the impedance formula for a capacitor (1/jwC), with DC, the frequency is zero and as a result omega is zero. If omega is zero, impedance essentially becomes infinite.

Question 13

What is the drain source given as in the TRIODE region?

Answer 13

gn is called the mosfet transconductance

Question 14

What happens to a MOSFET, …, and Vds keeps increasing?

Answer 14

The depth of the n-channel will become zero at the drain end.

This is known as channel pinch off.

Ids continues to flow, however, further increases in V_DS will not change Ids.

Mosfet is said to be in SATURATION REGION.

The drain source current is given by:

Question 15

With potentiometer biasing with source resistor: what value is Vs usually for large output voltage swing.

Answer 15

Usually within 1 and 3v