Test: Bias Compensation - UPSC MCQ

 Usually, the negative feedback is used to produce a stable operating point. But it reduces the voltage gain of the circuit. This sometimes is intolerable and should be avoided in some applications. So, the biasing techniques are used.

Compensation techniques refer to the use of temperature sensitive devices such as thermistors, diodes, transistors, sensistors etc to compensate variation in currents. Sometimes for excellent bias and thermal stabilization, both stabilization and compensation techniques are used.

For germanium transistor, changes in I_CO with temperature contribute more serious problem than for silicon transistor. On the other hand, in a silicon transistor, the changes of V_BE with temperature possesses significantly to the changes in I_C.

A diode is used as the compensation element used variation in V_BE and I_CO. The diode used is of the same material and type as that of transistor. Hence, the voltage across the diode has same temperature coefficient as V_BE of the transistor.

In this method, diode is used for the compensation in variation of I_CO. The diode used is of the same material and type as that of transistor. Hence, the reverse saturation current I_O of the diode will increase with temperature at the same rate as the transistor collector saturation current I_CO.

The thermistor has a negative temperature coefficient of resistance. It means, its resistance decreases exponentially with increasing T. The thermistor RT is used to minimize the increase in collector current.

: r_o=∆V_CE/∆I_C
=3/0.3m=10kΩ.

 The ratio of change in collector base voltage (∆V_CB) to resulting change in collector current (∆I_C) at constant emitter current (I_E) is defined as output resistance. This is denoted by r_o.

When no signal is applied, the ratio of collector current to emitter current is called dc alpha, αdc of a transistor. α_dc=-I_C/I_E. It is the measure of the quality of a transistor. Higher is the value of α, better is the transistor in the sense that collector current approaches the emitter current.

The purpose of the biasing is to switch on the BJT to work in the active region such that the DC collector current remains constant or Quiescent current or zero signal current independents of:

• β (beta)
• Temperature
• Load variations

The commonly used methods are

• Fixed base bias or Fixed bias
• Biasing with the emitter resistor
• Self - bias or Voltage divider bias

The stabilization occurs due to negative feedback action.

• The negative feedback, although improves the stability of the operating point, 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.

Diode compensation for Instability

These are the circuits that implement compensation techniques using diodes to deal with biasing instability. 

There are two types of diode compensation methods. They are −

• Diode compensation for instability due to V_BE variation
• Diode compensation for instability due to I_CO variation

Diode Compensation for Instability due to V_BE Variation

Diode Compensation for Instability due to I_C0 Variation

Other compensation

Other compensation techniques refer to the use of temperature-sensitive devices such as diodes, transistors, thermistors, Sensistors, etc. to compensate for the variation in currents.

There are two popular types of circuits in this method, one using a thermistor and the other using a Sensistor.

Thermistor Compensation

• A thermistor is a temperature-sensitive device.
• It has a negative temperature coefficient.

Sensistor Compensation

A Sensistor is a heavily doped semiconductor that has a positive temperature coefficient.