Transistor as an amplifier
Both types of transistor amplifiers operate using AC signal inputs that vary between a positive value and a negative value in order to allow a way to “preset” the amplifier circuit to operate between these two peak and average values. Using a process known as Biasing, this is done. In the amplifier design, biasing is very important as it determines the appropriate operating point for the transistor amplifier ready to receive signals, thus reducing any distortion of the output signal.
We also saw that a static or DC load line can be drawn onto these output characteristics curves to display all possible transistor operating points from full “ON” to full “OFF” and Where the quiet operating point or Q-point of the amplifier can be located.
The goal of any small signal amplifier is to amplify all the input signal with the minimum amount of distortion possible to the output signal, i.e. the output signal must be an exact representation of the input signal, but only larger (amplified).
The operating quiescent point must be correctly selected to achieve low distortion when used as an amplifier. This is in reality the amplifier’s DC operating point and its location can be determined by an appropriate biasing arrangement at any point along the load line.
For this Q-point, the best possible position is as close as reasonably possible to the center position of the load row, resulting in a Class A type amplifier function, i.e. Vce = Vcc 1/2. Consider the following circuit of the Standard Emitter Amplifier.
The Common Emitter Amplifier Circuit
Another type of biasing system uses two resistors as a potential divider network with their center point providing the transistor with the required base bias voltage.
This method of biasing the transistor greatly reduces the effects of changing Beta, (β) by maintaining the Base bias at a constant level of steady voltage allowing for maximum stability. The quiescent Base voltage (Vb) is defined by the potential divider network generated by the two resistors, R1, R2 and Vcc power supply voltage as shown with the current flowing through both resistors.
Then the maximum resistance RT is equal to the current R1 + R2 as I= Vcc / RT. The voltage level produced at the junction of the R1 and R2 resistors holds the constant base voltage (Vb) below the supply voltage.
Then the potential divider network used in the standard emitter amplifier system divides the supply voltage in proportion to the resistance. The basic voltage divider equation below can be used to measure this bias reference voltage easily:
Transistor Bias Voltage
The same supply voltage (Vcc) also determines the peak current of the collector, Ic when the transistor is completely turned ON Vce = 0. The transistor’s Base Current Ib is derived from the transistor’s Collector Current, Ic and DC Current Gain Beta, β.
Beta is sometimes referred to as hFE in the common emitter configuration as the transistors forward current gain. Beta does not have units as it is a fixed combination of the two currents, Ic and Ib, so a small change in the base current can cause a big change in the current of the Collector.
One last thing about Beta. For example, transistors of the same form and part number can vary greatly in their Beta value, the BC107 NPN Bipolar transistor has a DC current gain Beta value of between 110 and 450 (data sheet value) as Beta is a function of transistor construction and not of its operation.
As the base / emitter junction is forward-based, Ve will be a junction voltage drop that is different from the base voltage. If the voltage is known throughout the Emitter resistor then the Emitter current can be measured easily using Ohm’s Law. The current of the Collector, Ic can be approximated as it is almost the same value as the current of the Emitter.
Completed Common Emitter Circuit
Amplifier Coupling Capacitors
C1 and C2 are used as Coupling Capacitors in the Standard Emitter Amplifier circuits to isolate the AC signals from the DC bias voltage. It means that no additional amplifier stages will influence the bias condition set up for the circuit to work correctly, as the capacitors will only pass AC signals and block any DC element. On the biasing of the following steps, the incoming AC signal is then superimposed. CE is also included in the Emitter leg circuit as a bypass condenser.
This capacitor is essentially an open circuit element for DC bias conditions, which ensures that the biasing currents and voltages are not affected by inserting the capacitor to maintain good Q-point stability.
However because of its response, this parallel connected bypass capacitor effectively becomes a short circuit at high frequency signals to the Emitter resistor. Thus only RL plus a very small internal resistance acts as the transistors load up to their maximum increase in voltage gain. Generally, the CE bypass capacitor value is chosen to provide a response of at most 1/10th of the RE value at the lowest frequency of the operating signal.
when using a Transistor as an Amplifier
Instead, to sum up. In its Collector circuit, the Common Emitter Amplifier circuit has a resistor. The current that flows through this resistor produces the amplifier’s voltage output. The value of this resistor is chosen so that Q-point lies halfway along the load line of the transistors at the quiescent operating point of the amplifiers.
By Useing two resistors as a potential divider network, the transistor base used in a common emitter amplifier is biased. This type of biasing system is widely used in the design of bipolar transistor amplifier circuits and reduces the effects of varying beta, (β) by keeping the base bias at a steady voltage constant. This form of bias provides the highest level of stability.
In this case the voltage gain is -RL / RE, a resistor can be included in the emitter leg If there is no external resistance to the Emitter, the amplifier’s voltage gain is not infinite as there is a very low internal resistance, Re in the Emitter leg The internal resistance value is 25mV / IE.
We will look at the Junction Field Effect Amplifier commonly called the JFET Amplifier in the next section on transistor amplifiers. The JFET is used in a single stage amplifier loop, like the transistor, which makes it easier to understand. We could use many different types of field effect transistor, but the simplest to understand is the junction field effect transistor, or JFET, which has a very high input impedance, making it ideal for amplifier circuits.
Also Read – Transistor as a switch
Also Read – JFET as an amplifier