# The problem of Bjt Biasing techniques.

Discussion in 'Homework Help' started by Rhýthm Khanna, Feb 17, 2016.

1. ### Rhýthm Khanna Thread Starter New Member

Feb 8, 2016
1
0
Mods Edit:
This thread was split from -- Bjt Biasing techniques.

This given circuit looks like a combo. R1 and R2 are used as a voltage divider for providing biasing for class A operation. The Re resistor is used for emitter biasing.
Am I going right with this? Please let me know.
Also, I do not understand the usage of 2 input sources, one with the Rinput and the other one with input pin. Plus the use of the equation Rc=R3||RL is somewhat confusing as they do not seem to be forming a parallel network of resistors.
Will be eagerly waiting for a reply.

2. ### MrAl Distinguished Member

Jun 17, 2014
2,553
515
Need to show schematic

3. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Look closely at the quote in post 1, a schematic is embedded within.

All signals and batteries have a resistance included, I was showing them for demonstration purposes. It is a model after all. The input and output shown is where you would look with your instrumentation, be it an Oscope or a meter.

The transistor is a dynamic resistance, while R3 is fixed. It helps sets the output impedance of the amplifier, and interacts with the load. R3 is still part of the AC signal path.

thumb2 likes this.
4. ### dl324 Distinguished Member

Mar 30, 2015
3,377
651
Welcome to AAC! And getting your first thread hijack out of the way...
Same here; insufficient context.
This is the Homework Help forum; you need to show your work or ask questions if you're stuck.

5. ### Russmax Member

Sep 3, 2015
81
12
That is correct, or at least the designer hopes it is correct. There is a voltage loop formed by Vcc, R1, R2, Vbe, and Re. This loop is used to calculate Vb, Ve, and Ib. Remember, if Ic= β*Ib, then Ie=(β+1)*Ib.
Only the drawn source, with Rinput, is used. The input pin is test point.
Think of a circuit like this as 2 different circuits, a DC circuit with the capacitors open and an AC circuit with capacitors shorted and supply voltages = GND. The DC circuit is for calculating bias voltages and currents. Vcc, β, Vbe, Ic, Ib are used to analyze the DC circuit. The AC circuit is used for calculating voltage gain, Rin and Rout at AC frequencies (in the middle of the circuit's bandwidth). Small signal parameters βac, Rπ, and Ro apply to this circuit, which can be redrawn using the small-signal model.

In the AC circuit, C2 is shorted and Vcc=GND, making R3 parallel with RL.
ALWAYS keep these 2 circuits, DC and AC, separate in your mind. You must do the DC analysis 1st, because the small signal parameters are calculated from the DC bias current, and also to verify the transistor is not in saturation or cutoff.

Hope this helps.