Transistors

MrChips

Joined Oct 2, 2009
30,708
In many electronics circuit analyses, we look at the DC analysis and the AC analysis.

The DC base bias is affected primarily by the voltage divider composing of R1 and R2.
However, since RE (5kΩ) is going to determine the voltage at the emitter, this must be taken into consideration.
The DC analysis establishes the operating point (also known as the Q-point) of the transistor circuit.

What is AC analysis?
Suppose the DC voltage at the base terminal is VB.
What if we were to increase this base voltage ever so slightly by an amount we write as ΔV.
So the new base voltage is now VB + ΔV.
We compute a new determination of how this ΔV changes the operating current and voltages of the circuit.
We are interested in the change of current and voltage, not the absolute current and voltage.
This is AC analysis.

So what is the purpose of C1?
C1 allows the change ΔV to reach the base junction of the circuit regardless of the DC voltage at Vin.

C1 is called a DC blocking capacitor.
C1 is called an AC coupling capacitor.
C1 in conjunction of R1, R2, and the input impedance of the rest of the circuit constitute a high-pass filter. It passes high frequency signals and blocks low frequency signals.
Thus, you can call C1 by three different names. The function remains the same.

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MrChips

Joined Oct 2, 2009
30,708
Ok so does the current that turn on the base of the transistor coming frm the ac signal or from vcc( fixed bais)
Yes and no. Neither or both.
It depends on the design of the circuit.
There are different classes of amplifiers, mainly class A, AB, B, C, D, E.

In a class-A amplifier, the Q-point is in the linear region without any AC signal. Hence the transistor is already conducting.
Adding the AC signal moves the transistor from less conducting to more conducting, straddling both sides of the Q-point.
Examine the drawing on post #44.
 

Audioguru again

Joined Oct 21, 2019
6,673
The DC base bias turns on the transistor about halfway from fully on to fully off. Then the AC input signal varies the base voltage a little which causes the transistor to turn on a little or a lot and then turn off a little or a lot. The amplitude of the base input signal and the designed voltage gain of the circuit determines the amplitude of the collector output signal.

The transistor circuit in post #59 and post #62 does not have the output from the transistor's collector, instead the output is from the emitter. Then the transistor circuit is called an Emitter follower than has the same output amplitude as the input signal amplitude.
 

Wolframore

Joined Jan 21, 2019
2,609
VB approximately 5v, VE is 4.3v, this is your DC bias. You have approx 860 uA flowing through RE.

Your Av is Slightly less than 1. Edit: misread the resistor values. You’re getting current gain of approx 13.
 
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