Electronics: CE Transistor Circuit DC Bias. Applying Thevenin's Theorem

This article explains the application of Thevenin's Theorem to the popular type of BC bias shown in the analog common emitter transistor circuit in Figure One. Note that figure one shows the AC voltage source as well as the DC voltage source.

For DC calculations the capacitors can be considered as open circuits. This circuit is shown in Figure Two. The resistor R5 is not in the circuit because the capacitor is an open circuit to DC levels. The capacitors are blocking capacitors. They are used to block the DC voltage from reaching certain components in the circuit. For example: The capacitor C1 blocks the DC voltage across R4 from reaching the AC source labeled Vac. Hence, for DC calculations: The AC voltage source is not in the circuit.

We can apply Thevenin's theorem to the DC bias circuit in order to simplify our bias calculations.

Here is the introduction to Thevenin's Theorem originally written in my article "Electrical Theory: Thevenin's Theorem" at the URL

http://www.associatedcontent.com/article/1715357/basic_electrical_theory_thevenins_theorem.html?cat=15

"Introduction to Thevenin's Theorem

Thevenin's theorem states that any linear DC network consisting of voltage sources and/or current sources, linear components and a two terminal output can be replaced by a single source and a single resistor. The network must be a two terminal bilateral network.

Generally speaking, the load resistor would be connected across the two terminals. These two terminals would be the output of the network. Input terminals does not exist because the network does not depend on an external electrical source. Networks that depend on an external electrical source have two input terminals and two output terminals.

Bilateral means a given current flow in either direction causes the same voltage drop.

A network containing a diode is not a bilateral network because the diode has a very low resistance when current is flowing in one direction and a very high resistance when current is flowing in the other direction."

First we must isolate the circuit we want to apply Thevenin's Theorem to. That circuit would comprise of the voltage source Vcc and the resistors R3 and R4 as shown in Figure 3a. The output of the circuit is across R4.

The first step is to solve for the voltage output across R4. This voltage is known as Thevenin's voltage and will be labeled Vth.

R3 and R4 form a voltage divider. Hence the voltage Vth across R4 is

Vth = (R4/(R3+R4) )*Vcc

Next we solve for the Thevenin Resistance Rth. The Thevenin Resistance is the resistance measured across R4 with the DC voltage source shorted as shown in Figure 3b. Hence the resistors R3 and R4 are in parallel.

The Thevenin resistance Rth is

Rth = R3*R4/(R3+R4)

Hence we have our Thevenin's circuit which is shown in figure 4. Compare this circuit to the circuit in Figure One.

Also note that the DC bias in the circuit of Figure 4 matches the DC bias in Figure One of my article on Thevenin's Theorem at

http://www.associatedcontent.com/article/1715357/basic_electrical_theory_thevenins_theorem.html?cat=15

Hence, for further DC calculations, refer to the article at the URL above.

The next article will cover the AC circuit calculations and the AC load line.

References:

I have a Bachelor of Science In Electrical Engineering and worked as an Electronics Technician.

Electronic Principles: Third Edition
ISBN 0-07-039912-3