Bias Circuit Thermal Stability:

V_BE and I_CBO Variations – Many transistor circuits are required to operate over a wide temperature range. So, another aspect of bias circuit stability is Bias Circuit Thermal Stability, or how stable I_C and V_CE remain when the circuit temperature changes.

Measures to deal with the effects of h_FE variations have already been discussed. These apply whether the different h_FE values are due to temperature changes or to h_FE differences from one transistor to another.

The base-emitter voltage (V_BE) and the collector-base reverse, saturation current (I_CBO) are the two temperature-sensitive quantities that largely determine the Bias Circuit Thermal Stability of a transistor circuit, [see Fig. 5-48(a)]. The base-emitter and collector-base pn-junctions have the temperature characteristics. For a silicon transistor, V_BE changes by approximately -1.8 mV/°C, and I_CBO approximately doubles for every 10°C rise in temperature. These effects are illustrated by the characteristics in Fig. 5-48(b) and (c).

An increase in I_CBO causes I_C to be larger, and the I_C increase raises the collector-base junction temperature. This, in turn, results in a further increase in I_CBO. The effect is cumulative, so that the end result might be a substantial collector current increase. This could produce a significant shift in the circuit Q-point, or in the worst case, I_C might keep on increasing until the transistor collector-base junction overheats and burns out. This effect is known as thermal runaway. Measures taken to avoid thermal runaway are similar to those required for good bias stability against h_FE spread.

Changes in V_BE may also produce significant changes in I_C and consequently in the circuit Q-point. However, because of the possibility of thermal runaway, I_CBO changes are the most important. The Bias Circuit Thermal Stability is assessed by calculating a stability factor.

Stability Factor:

The stability factor (S) of a circuit is the ratio of the change in collector current to the change in collector-base leakage current.

The value of S depends on the circuit configuration and on the resistor values. The minimum values of S is 1. This means that if I_CBO increases by 1 μA I_C will increase by 1 μA. If a circuit has an S of 50, then ΔI_C = 50 x ΔI_CBO. A stability factor of 50 (or larger) is considered poor, while a factor of 10 or less is considered good.

An equation for the stability factor of a bias circuit can be derived by writing an equation for the circuit I_C and investigating the effect of I_CBO change. The stability factors for the three basic bias circuit types (reproduced in Fig. 5-49) can be shown to be:

For base bias,

For collector-to-base bias,

For voltage divider bias,

The change in I_CBO over a given temperature range can be calculated by recalling that I_CBO doubles for every 10°C increase in temperature. The temperature change (ΔT) is divided by 10 to give the number of 10°C changes (n). If the starting level of collector-base leakage current is I_CBO(1), the new level is,

The I_CBO change and the circuit stability factor can be used to determine the change in I_C, (Eq. 5-18). Then the resulting V_CE change can be investigated.

Effect of V_BE Changes:

Consider the voltage divider bias circuits in Fig. 5-51(a) and (b), which each have V_CC = 12 V and I_C = 1 mA. From Eq. 5-9,

Assuming that V_B remains substantially constant, an equation for I_C change with V_BE change can be written:

As discussed already, the emitter resistor voltage should typically be selected as 5 V, (V_E >> V_BE). This is to ensure that I_C is not significantly affected by changes in V_BE. Thus, the circuit in Fig. 5-51(a) (with V_E ≈ 5 V) has greater stability against V_BE changes than the one in Fig. 5-51(b) which was designed for V_E = V_CC/10,(V_E = 1.2V).

Diode Compensation:

The use of a diode to compensate for V_BE changes is illustrated in Fig. 5-52. In this case,

When V_BE changes by ΔV_BE, the diode voltage changes by an approximately equal amount (ΔV_D1). ΔV_BE and ΔV_D1 tend to cancel each other, leaving I_C largely constant at,

Base bias and collector-to-base bias are less affected by V_BE changes than voltage divider bias. This can easily be demonstrated by considering the equations for the base current in each case.