The bootstrap circuit

The bootstrap circuit

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On: 05 Agu, 2020

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With many transistor circuits in the basic emitter circuit, the static operating point is set in a stable manner with the aid of a base voltage divider and a low-ohm emitter resistor. Both affect the input impedance of the circuit. From the input ...

With many transistor circuits in the basic emitter circuit, the static operating point is set in a stable manner with the aid of a base voltage divider and a low-ohm emitter resistor. Both affect the input impedance of the circuit. From the input point of view, the value of the emitter resistance must be multiplied by the current amplification factor. The resistors of the base voltage divider are parallel in the AC equivalent circuit and reduce the input impedance. Good stabilization is achieved with a high cross-current factor, which is achieved by a relatively low-resistance lower divider resistance. This value then ultimately determines the value of the input impedance, which for emitter circuits is often less than 10 kiloohms.

If the signal from a high-resistance source such as a condenser microphone or a parallel resonant circuit is amplified with this circuit concept, the source is heavily loaded by a low input or connection impedance. The usable signal amplitude becomes very small, the open circuit voltage of the source collapses. In the case of resonant circuits, the circular quality also deteriorates and the bandwidth increases. The bootstrap circuit is a special input circuit of the amplifier with which these disadvantages are largely eliminated. (Bootstrap, English boot loop, serves as an aid to putting on and is translated here as a control aid).

The base direct current no longer reaches the base directly from the voltage divider R1, R2, but via an additional resistor R3. Through this resistance, the signal current i3 also flows to ground via R2. The signal source is loaded by around 35 kΩ. A coupling capacitor C, the bootstrap capacitor, is now inserted between the emitter and the base of R3. It couples the signal voltage uE from the emitter resistor to the base point. Part of the signal input voltage ue and the voltage uE coming from the emitter are in phase at R3. The AC voltage difference u3 remaining across the resistor is very small if dimensioned correctly. The signal current i3 flowing to ground is therefore also very small. The bootstrap capacitor has consequently increased the input resistance for the signal.

figure 1

figure 1

The circuit has a voltage gain of 20 dB. The oscillograms show the relationships between the signal voltage at the additional resistor R3 above without the bootstrap capacitor and below with it. The alternating current i3 is reduced from 10.6 μA without to 1.3 μA with a bootstrap capacitor. The input impedance was determined in the simulation using the half output voltage method. The bootstrap circuit delivers the significantly higher value. Since the bootstrap capacitor affects the lower limit frequency, it should have a high capacitance. In this circuit, the lower limit frequency of around 1 Hz was measured with 1 μF.

The emitter resistance required for the bootstrap circuit reduces the maximum achievable voltage gain. However, it increases and improves the input impedance of the circuit significantly. In order for it to remain effective, the emitter resistance to increase the voltage gain must not be canceled by a parallel-connected emitter capacitor. The bootstrap circuit is often also used in the collector circuit, where the emitter resistor cannot be dispensed with.

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