Biasing of a Transistor

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Biasing of a Transistor

Biasing a Transistor amplifier is the process of setting the DC operating voltage and current to the correct level so that any AC input signal can be amplified correctly by the transistor. The correct bias point of the transistor is generally somewhere between the two extremes of operation with respect to it being either “fully-ON” or “fully-OFF” along the load line. This central operating point is called the “Quiescent Operating Point”, or Q-point for short.

When a bipolar transistor is biased so that the Q-point is near the middle of its operating range, that is approximately halfway between cutoff and saturation, it is said to be operating as a Class-A amplifier. This mode of operation allows the output current to increase and decrease around the amplifiers Q-point without distortion as the input signal swings through a complete cycle. In other words, the output current flows for the full 360^o of the input cycle.

So how do we set this Q-point biasing of a transistor? – The correct biasing of the transistor is achieved using a process know commonly as Base Bias. The function of the “DC Bias level” or “no input signal level” is to correctly set the transistors Q-point by setting its Collector current ( I_C ) to a constant and steady state value without an input signal applied to the transistors Base. This steady-state or DC operating point is set by the values of the circuits DC supply voltage ( Vcc ) and the value of the biasing resistors connected the transistors Base terminal.

Since the transistors Base bias currents are steady-state DC currents, the appropriate use of coupling and bypass capacitors will help block bias current setup for one transistor stage affecting the bias conditions of the next. Base bias networks can be used for Common-base (CB), common-collector (CC) or common-emitter (CE) transistor configurations. In this simple transistor biasing tutorial we will look at the different biasing arrangements available for a Common Emitter Amplifier.

Base Biasing a Common Emitter Amplifier

One of the most frequently used biasing circuits for a transistor circuit is with the self-bias of the emitter-bias circuit where one or more biasing resistors are used to set up the initial DC values of transistor currents, ( I_B ), ( I_C ) and ( I_E ). This is achieved either using a feed back resistor or by using a simple voltage divider circuit to provide biasing voltage. The following are five examples of transistor Base bias configurations from a single supply ( Vcc ).

Current Biasing a Transistor

A two resistor biasing network is used to establish the initial operating region of transistor with a “fixed current bias”. The Emitter diode of the transistor is forward biased by applying the required positive Base bias voltage via the current limiting resistor R_B.

Assuming a standard bipolar transistor, the forward Base-emitter voltage drop will be 0.7V. Then the value of I_B is simply: (V_CC – V_BE)/I_B. With this type of biasing method the biasing voltages and currents do not remain stable during transistor operation.
 

Feedback Biasing a Transistor

This transistor biasing configuration again requires only two resistors. The Collector feedback configuration ensures that the transistor is always biased in the active region regardless of the value of Beta (β). The Base bias is derived from the Collector voltage.

If the Collector current increases, the Collector voltage drops, reducing the Base drive and thereby automatically reducing the Collector current. The transistors stability using this type of bias network is generally good for most amplifier designs.
 
 

Dual Feedback Biasing a Transistor

 
Adding an additional resistor to the Base bias network of the previous configuration improves stability even more with respect to variations in Beta, (β) by increasing the current flowing through the Base bias resistors. The current flowing through R_B1 is generally set at a value equal to about 10% of Collector current, I_C.

One of the advantages of this type of biasing configuration is that the resistors provide both biasing and Rf feedback at the same time.
 
 
 
 

Voltage Divider Biasing a Transistor with Compensation

The common Emitter transistor is biased using a voltage divider network. The name of
this biasing configuration comes from the fact that the two resistors R_B1and R_B2 are connected to the transistors Base terminal across the supply. This voltage divider configuration is the most widely used biasing method, as the Emitter diode of the transistor is forward biased by the voltage dropped across resistor R_B2.

To calculate the voltage developed across resistor R_B2 and therefore the voltage applied to the Base terminal we simply use the voltage divider formula. The current flowing through resistor R_B2 is generally set at 10 times the value of the required Base current I_B so that it has no effect on the voltage divider current.

Dual Feedback Biasing a Transistor with Emitter Compensation

 
This type of biasing configuration uses both Emitter and Collector-base feedback to stabilize the Collector current. Resistor values are generally set so that the voltage drop across Emitter resistor R_E is approximately 10% of V_CC and the current flowing through resistor R_B1 is 10% of the Collector current I_C.

This type of transistor biasing configuration works best at relatively low power supply voltages.
 
 
 
 

The goal of biasing a transistor is to establish a known Q-point in order to work efficiently and produce an undistorted output signal. Correct biasing of the transistor also establishes its initial AC operating region with practical biasing circuits using either a two or four-resistor bias network. In bipolar transistor circuits, the Q-point is represented by (VCE, IC) for the NPN transistors or (VEC, IC) for PNP transistors. The stability of the Base bias network and therefore the Q-point is generally assessed by considering the Collector current as a function of both Beta (β) and temperature.

Here we have looked briefly at five different configurations for “biasing a transistor” using resistive networks. But we can also bias a transistor using either silicon diodes, zener diodes or active networks all connected to the Base terminal of the transistor or by biasing it from a dual power supply.