Bias Stabilization
Overview
This will be useful in understanding bias compensation and transistor stability i.e. operating point stability.
Bias compensation
The stabilization occurs due to negative feedback action. The negative feedback, although improves the stability of operating point, it reduces the gain of the amplifier.
As the gain of the amplifier is a very important consideration, some compensation techniques are used to maintain excellent bias and thermal stabilization. Let us now go through such bias compensation techniques.
Diode Compensation for Instability
These are the circuits that implement compensation techniques using diodes to deal with biasing instability. The stabilization techniques refer to the use of resistive biasing circuits which permit IB to vary so as to keep IC relatively constant.
There are two types of diode compensation methods. They are −
- Diode compensation for instability due to VBE variation
- Diode compensation for instability due to ICO variation
Let us understand these two compensation methods in detail.
Diode Compensation for Instability due to VBE Variation
In a Silicon transistor, the changes in the value of VBE results in the changes in IC. A diode can be employed in the emitter circuit in order to compensate the variations in VBE or ICO. As the diode and transistor used are of same material, the voltage VD across the diode has same temperature coefficient as VBE of the transistor.
The following figure shows self-bias with stabilization and compensation.
The diode D is forward biased by the source VDD and the resistor RD. The variation in VBE with temperature is same as the variation in VD with temperature, hence the quantity (VBE – VD) remains constant. So the current IC remains constant in spite of the variation in VBE.
Diode Compensation for Instability due to ICO Variation
The following figure shows the circuit diagram of a transistor amplifier with diode D used for compensation of variation in ICO.
So, the reverse saturation current IO of the diode will increase with temperature at the same rate as the transistor collector saturation current ICO.
I=VCC−VBER≅VCCR=ConstantI=VCC−VBER≅VCCR=Constant
The diode D is reverse biased by VBE and the current through it is the reverse saturation current IO.
Now the base current is,
IB=I−IOIB=I−IO
Substituting the above value in the expression for collector current.
IC=β(I−IO)+(1+β)ICOIC=β(I−IO)+(1+β)ICO
If β ≫ 1,
IC=βI−βIO+βICOIC=βI−βIO+βICO
I is almost constant and if IO of diode and ICO of transistor track each other over the operating temperature range, then IC remains constant.