Bipolar transistor biasing
For bipolar junction transistors (BJTs), the operating point is defined as the steady-state DC collector-emitter voltage (
A bias circuit may also include elements such as temperature-dependent resistors, diodes, or additional voltage sources, depending on the expected range of operating conditions.
In class-A amplifiers, the operating point is chosen such that the transistor stays in forward-active mode across the input signal's range.
The operating point is often set near the center of the forward-active region, allowing for equal positive and negative swing in the input signal.
An effective bias circuit establishes an operating point that remains stable even when certain parameters vary.
For BJTs, the following parameters can affect the operating point:[1] At constant current, the base-emitter voltage decreases by 2 mV (silicon) for each 1 °C rise in temperature (reference being 25 °C).
By the Ebers–Moll model, if the base-emitter voltage is held constant while the temperature rises, the base current (
And since Rb and the DC voltage source Vcc are constant, the base current Ib also doesn't vary significantly.
The common-emitter current gain of a transistor (specified as a range on its data sheet as hFE or β), allows us to obtain
However, one application of fixed bias is to achieve crude automatic gain control in the transistor by feeding the base resistor from a DC signal derived from the AC output of a later stage.
This configuration employs negative feedback to prevent thermal runaway and stabilize the operating point.
A lower base-resistor voltage drop reduces the base current
Because an increase in collector current with temperature is opposed, the operating point is kept stable.
Due to the gain reduction from feedback, this biasing form is used only when the trade-off for stability is warranted.
This resistor introduces negative feedback that stabilizes the operating point.
Advantages: The circuit has the tendency to stabilize operating point against changes in temperature and β-value.
Disadvantages: Usage: The feedback also increases the input impedance of the amplifier when seen from the base, which can be advantageous.
Due to the above disadvantages, this type of biasing circuit is used only with careful consideration of the trade-offs involved.
By proper selection of resistors R1 and R2, the operating point of the transistor can be made independent of β.
By manipulating the resistors in certain ways you can achieve more stable current levels without having β value affect it too much.
The standard voltage divider circuit discussed above faces a drawback – AC feedback caused by resistor Re reduces the gain.
This can be avoided by placing a capacitor (Ce) in parallel with Re, as shown in circuit diagram.
[clarification needed] The negative supply Vee is used to forward-bias the emitter junction through Re.
Advantages: Disadvantages: Class B and AB amplifiers employ 2 active devices to cover the complete 360 deg of input signal flow.
Each transistor is therefore biased to perform over approximately 180 deg of the input signal.
Class B bias is when the collector current Ic with no signal is just conducting (about 1% of maximum possible value).
Class-AB bias is when the collector current Ic is about 1⁄4 of maximum possible value.
If the output transistors conduct too much, they can easily overheat and destroy themselves, as the full current from the power supply is not limited at this stage.
A common solution to help stabilize the output device operating point is to include some emitter resistors, typically an ohm or so.
Calculating the values of the circuit's resistors and capacitors is done based on the components employed and the intended use of the amplifier.
