4.2 Individual Homework Give numbered responses to the questions below in a shor

Question

4.2 Individual Homework Give numbered responses to the questions below in a short document (min. 300 words). 1. In this lab you found pnd output of your common-emitter amplifier. Explain, in your own words, why these values are of practical interest when designing circuits. 2. What happens to your gain, generally, when you attach a load resistance? If you wanted to preserve 90% of your measured no-load gain when you hook up a load, what load resistance values would be acceptable? 3. Assuming you wanted the largest undistorted signal amplitude possible, what DC collector voltage should you have designed this circuit to have? Explain why

Explanation / Answer

Ans 1)

The Input Impedance of an amplifier defines its input characteristics with regards to current and voltage looking into the amplifiers input terminals.

But in most applications, common emitter and common collector amplifier circuits generally have high input impedances.

Some types of amplifier designs, such as the common collector amplifier circuit automatically have high input impedance and low output impedance by the very nature of their design. Amplifiers can have high input impedance, low output impedance, and virtually any arbitrary gain, but were an amplifiers input impedance is lower than desired, the output impedance of the previous stage can be adjusted to compensate or if this is not possible then buffer amplifier stages may be needed. the input impedance of a common emitter amplifier can be found by shorting out the supply voltage and treating the voltage divider biasing circuit as resistors in parallel. The impedance “seen” looking into the divider network (R1||R2) is generally much less than the impedance looking directly into the transistors Base, (RE+ re) as the AC input signal changes the bias on the Base of the transistor controlling the current flow through the transistor. The output impedance of a common emitter stage is just equal to the collector resistor in parallel with the load resistor (RC||RL) if connected otherwise its just RC. The voltage gain, Av of the amplifier is dependant upon RC/RE.

Then we can see that the input and output impedances of an amplifier can play an important role in defining the transfer characteristics of an amplifier with regards to the relationship between the output current, Ic, and the input current, Ib. Knowing an amplifiers input impedance can help to graphically construct a set of output characteristics curves for the amplifier.

Ans 2)

Suppose you have a common emitter transistor amplifier driving the load of say 4 ohms.

The gain of a transistor is directly proportional to the collector resistance that means higher you keep it better the gain ( keeping in mind that increasing RC too much can lead to a low IC sat and thus drive your transistor in saturation if biasing arrangement is not taken care of).

Generally, this collector resistance is in K ohms range, for example, say 4.7K

So what happens is that this load of 4 ohms is attached to this. It comes in parallel with the RC=4.7K and drastically reduces the effective collector resistance and hence reduces the gain. Now what has happened is that your output voltage is decreased due to the loading effect.
Suppose you are using a feedback mechanism in which the output voltage is fed back to input side then due to loading effect even your effective input is decreasing. Since the output voltage is proportional to input voltage also ( Vout = Gain*Vin) so loading effect us cumulatively decreases the voltage supply.

Ans 3)

Distortion of the output signal waveform may occur because:

This means then that during the amplification process of the signal waveform, some form of Amplifier Distortion has occurred.

The value of voltage depends on the circuit.

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