The forward bias means the P-region of the transistor is connected to the positive terminal of the supply and the negative region is connected to the N-terminal and in reverse bias just opposite of it has occurred. The input signal or weak signal is applied across the emitter base and the output is obtained to the load resistor R C which is connected in the collector circuit. The DC voltage V EE is applied to the input circuit along with the input signal to achieve the amplification.
The DC voltage V EE keeps the emitter-base junction under the forward biased condition regardless of the polarity of the input signal and is known as a bias voltage. The emitter junction bias is provided by battery 1, and the collector junction bias is provided by battery 2.
Figure 5. Forward and reverse bias in a PNP transistor amplifier circuit. Single Battery Circuit. Two voltage sources have been used in all circuits discussed thus far. One source was used for the forward biasing of the emitter junction, and the other source was used for the reverse biasing of the collector junction.
There is no need for two batteries. The amplifier in Figure 6 uses only one battery. Figure 6. This circuit uses a single power source. Biasing Methods. The fixed bias method is shown in Figure 7. Notice that the resistor R B has been omitted. This circuit sets a constant base current. This bias is used for switching circuits. A switch is inserted in the base to control current through the emitter-collector.
Figure 7. Fixed bias method for connecting a transistor. The single battery bias scheme is another common method for biasing transistors, Figure 8. Figure 8. The collector current changes very slightly with the change in collector-emitter voltage.
The load is connected at the collector of a transistor and for a single-stage amplifier, the output voltage is taken from the collector of the transistor and for a multi-stage amplifier, the same is collected from a cascaded stages of transistor circuit. By definition, it is the total load as seen by the a. In case of single stage amplifiers, the effective collector load is a parallel combination of R C and R o.
In a multi-stage amplifier i. Effective collector load becomes parallel combination of R C , R o and R i i. The gain in terms of current when the changes in input and output currents are observed, is called as Current gain.
The gain in terms of voltage when the changes in input and output currents are observed, is called as Voltage gain. As the load gets larger which, conversely, means the resistance is lower the output of the voltage divider circuit drops. But the voltage output of the emitter follower remains steady, regardless of what the load is.
Bigger loads can't "load down" an emitter follower, like they can circuits with larger output impedances. We'll talk about common base to provide some closure to this section, but this is the least popular of the three fundamental configurations.
In a common base amplifier, the emitter is an input and the collector an output. The base is common to both. Common base is like the anti-emitter-follower. It's a decent voltage amplifier, and current in is about equal to current out actually current in is slightly greater than current out.
The common base circuit works best as a current buffer. It can take an input current at a low input impedance, and deliver nearly that same current to a higher impedance output. These three amplifier configurations are at the heart of many more complicated transistor amplifiers. They each have applications where they shine, whether they're amplifying current, voltage, or buffering.
We could go on and on about the great variety of transistor amplifiers out there. Here are a few quick examples to show off what happens when you combine the single-stage amplifiers above:. The Darlington amplifier runs one common collector into another to create a high current gain amplifier. Voltage out is about the same as voltage in minus about 1. The Darlington pair is a great tool if you need to drive a large load with a very small input current.
A differential amplifier subtracts two input signals and amplifies that difference.
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