A technique used in a variety of applications in which a capacitor – the bootstrap capacitor – is used to provide 100% positive feedback for alternating currents across an amplifier stage of unity gain or less. Bootstrapping is used for control of the output signals by using the positive feedback to control the conditions in the input circuit in a desired manner.

Bootstrapping is commonly used in circuits that generate a linear timebase, particularly in a sawtooth generator. A simple sawtooth generator consists of a capacitor that is charged by means of an input load resistor and discharged by a periodic step voltage. As the capacitor is charged, the voltage increases exponentially and as the voltage increases, that across the input load (and hence the charging current) drops correspondingly. The output is approximately linear provided that only a small portion of the charging characteristic is used. The linearity may be improved by using a bootstrap circuit to maintain a constant charging current. A typical circuit is shown in Fig. a. The output is taken from an emitter follower, capacitively coupled via the bootstrap capacitor C₁ to the input load resistor R. As the output voltage rises, the voltage at the node between R and R₁ also rises; the voltage across R and hence the charging current is therefore maintained substantially constant.

Bootstrapping may also be used in MOS logic circuits in order to optimize the voltage swing between the high and low logic levels. A typical bootstrapped circuit is shown in Fig. b. The output voltage V_o at point X is high when a low logic level is applied to the gate of the transistor T_s and is determined by the value of the gate and threshold voltages of the transistor T₁. In the absence of the bootstrap capacitor, C, and the load transistor, T_L, the gate voltage V_G1 of T₁ is equal to V_DD and

where V_T is the threshold voltage. If a load transistor T_L is present then

If V_G1 can be increased to a value greater than V_DD, then V_o can also rise to a maximum possible value equal to V_DD. Bootstrapping is used to achieve this effect. A bootstrap capacitor, C, is connected between points X and Y (Fig. b). As V_o rises from the low logic level, V_G1 also rises because of the positive feedback provided by C. This causes T_L to switch off and point B is thus isolated from the supply bus. V_G1 can therefore rise to a value greater than V_DD and as V_G1 increases, V_o also increases until it reaches V_DD. The total voltage swing at X is therefore optimized.

(a) Bootstrapping in a sawtooth generator

(b) Bootstrapping in MOS circuits

Bootstrapping is also used in high input impedance amplifiers, such as an emitter follower or a field-effect transistor stage, in order to maximize the a.c. input impedance. The a.c. input signal to the base or gate electrode can tend to flow through the bias resistors used to provide the d.c. bias for the base or gate unless bootstrapping is provided to prevent this effect.