![]() ![]() ![]() We have also seen that as the gain goes up the input impedance goes down from 15.8kΩ without it to 2.2kΩ with it. It also shows that the common emitter gain does not go to infinity when the external emitter resistor is shorted by the bypass capacitor at high frequencies but instead the gain goes to the finite value of R OUT/re. Then we can see that the inclusion of the bypass capacitor within the amplifier design makes a dramatic change to the voltage gain, Av of our common emitter circuit from 0.5 to 33. Then if we short out the 12 volt power supply, Vcc to ground because Vcc appears as a short to AC signals, we can redraw the common emitter circuit above as follows: Amplifier Circuit Model Where R EQ is the equivalent resistance to ground (0v) of the biasing network across the Base, and re is the internal signal resistance of the forward biased Emitter layer. The generalised formula for the AC input impedance of an amplifier looking into the Base is given as Z IN = R EQ||β(R E+ re). However when an AC signal is applied to the input, the characteristics of the circuit changes as capacitors act as short circuits at high frequencies and passes the AC input signal. The input capacitor, C1 acts as an open circuit and therefore blocks any externally applied DC voltage.Īt DC (0Hz) the input impedance ( Z IN) of the circuit will be extremely high. The DC bias circuit sets the DC operating “Q” point of the transistor. The generalised formula for the input impedance of any circuit is Z IN = V IN/I IN. ![]() Now we have the values established for our common emitter amplifier circuit above, we can now look at calculating its input and output impedance of amplifier as well as the values of the coupling capacitors C1 and C2. Thus the common emitter configuration produces a large voltage amplification and a well defined DC voltage level by taking the output voltage from across the collector as shown with resistor R L representing the load across the output. ![]() Then the direction of change of the Collector voltage is opposite to the direction of change on the Base, in other words, the polarity is reversed. A signal current into the Base causes a current to flow in the Collector resistor, Rc generating a voltage drop across it which causes the Collector voltage to drop. With no signal current flow into the Base, no Collector current flows, (transistor in cut-off) and the voltage on the Collector is the same as the supply voltage, Vcc. Power supply Vcc and the biasing resistors set the transistor operating point to conduct in the forward active mode. The so called classic common emitter configuration uses a potential divider network to bias the transistors Base. In this tutorial we will look at the bipolar transistor connected in a common emitter configuration seen previously. The amplifier itself can be connected in Common Emitter (emitter grounded), Common Collector (emitter follower) or in Common Base configurations. Then we can see that the input and output characteristics of an amplifier can both be modelled as a simple voltage divider network. 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. But in most applications, common emitter and common collector amplifier circuits generally have high input impedances. If it is too low, it can have an adverse loading effect on the previous stage and possibly affecting the frequency response and output signal level of that stage. The input impedance of an amplifier is the input impedance “seen” by the source driving the input of the amplifier. Input Impedance, Z IN or Input Resistance as it is often called, is an important parameter in the design of a transistor amplifier and as such allows amplifiers to be characterized according to their effective input and output impedances as well as their power and current ratings.Īn amplifiers impedance value is particularly important for circuit analysis especially when cascading individual amplifier stages together one after another in order to minimise distortion of the signal through the amplification circuit. ![]()
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