How the additional resistor can be use to reduce signal distortion? (See figure)?

Question

Figure:
http://www.geocities.com/h61pri_g19/images/bjt.jpg

Figure 1 without the additional resistor.
Figure 2 with the additional resistor (R5) attached.

Answer 1

R5 on Fig. 2 does not change the DC bias so it does not change the operating point of the transistor. It does, however attenuates the pass AC input signal, so it can reduce distortion especially if without it the input signal would overdrive the transistor into non-linear gain region.

Answer 2

Both circuits are terrible from a practical standpoint but adding the resistor in figure 2 will limit the gain a bit and keep the circuit a bit more linear.As it stands, no one would ever ship any real product with either circuit, They're both too sensitive to transistor characteristic variations. As far as distortion goes, by the way adding a resistor between the base and collector of the device would work the best. Shunt feedback is nice for minimizing distortion.

Answer 3

Neither of these cicuits will yeild reasonable performance, with fig. 2 only being marginally better. Adding resistance in the emitter will provide internal feedback, lowering distortion and adding thermal bias stability, but will also reduce overall gain. Another solution is to use fig. 2 but connect R4, the base bias resistor directly to the collector rather than Vcc. This will stabilize the bias Q-point as well as greatly improve the input impedance and reduce distortion due to feedback. This is the preferred method.

Answer 4

This makes an RC filter at the input of the transistor amplifier.
Most non common base amplifiers have high input impedance anyway so the resistor would have to be much larger than that.
f sub c = 1/2piRC for the 1/2 Power point.

Answer 5

R5 in the figure use to reduces or balance the Base current (Ib). Since Ic=beta x Ib collector current (Ic) controlled by Base current, where desire output transition controlled to desired level avoiding distortion to occur.

Answer 6

It minimizes the emitter curent variations by limiting the current. Since the emitter junction has a square-law V-I curve, driving it too far in either direction increases distortion by making the gain non-linear.


Doug