Bootstrapping in Electronics Circuits
Bootstrapping is a technique commonly used in electronic circuit design to increase the voltage level of a signal, generate high-impedance inputs, or implement a self-biasing circuit. It is a technique used in electronics circuit to increase the voltage available to a device, while using a supply voltage that is lower than the desired output voltage. It is often used to drive high-impedance circuits, such as gate inputs in MOSFETs, with a low-impedance voltage source. The idea is to use a fraction of the output voltage to "boost" the supply voltage to a higher level, which can then be used to drive the circuit. This is accomplished by using a capacitor connected in a feedback loop, which stores energy from the output voltage and provides it back to the input. Bootstrapping is commonly used in power amplifiers, operational amplifiers, and switched-mode power supplies, among other applications.
The bootstrapping technique involves a self-supporting feedback effect, allowing a circuit to function effectively despite limited resources. The goal is to increase the apparent input resistance value, Ri, to as high as possible. By using the bootstrapping method, the negative impact of bias resistors, which decrease the input resistance, can be minimized.
Bootstrapping with Bipolar Junction Transistor
The bootstrap technique was developed specifically for bipolar junction transistors (BJT) as it is not necessary for MOSFETs due to their naturally high input resistance caused by the gate oxide. It is especially useful in the common collector configuration. The bootstrap capacitor CF is connected in a loop between the output and input of the common collector amplifier as shown in the figure below(both circuits are equivalent).
The capacitor CF must act as a short circuit for AC signals and an open circuit for DC signals. This allows the AC feedback effect of resistor R3 without affecting the bias conditions. Therefore, the reactance of the capacitor (Xcp = 1/2 * pi * CF) must be lower than the resistance value of R3 at the lowest operating frequency of the amplifier.
Mathematically, it means,
CF >> 1/(2*pi*f*R3)
If Av = Vo/Vi is the amplifier gain,then the voltage and current across resistor R3 are:
Vr3=Vi-Vo=Vi-Av*Vi=Vi(1-Av)
and, Ir3=Vr3/R3=(Vi(1-Av))/R3
And the input resistance is,
Ri = Vi/Ii=Vi/Ir3=R3/(1-Av)
Since the voltage gain of a common collector amplifier is less than unity, the AC resistance of R3 acts like it has a much higher value than its actual value. An amplifier with bootstrap like the one shown above has the capability of enhancing the input resistance value by approximately two orders of magnitude.
Bootstrapping Applications
Some common applications of bootstrapping in electronics circuits include:
Amplifier circuits: Bootstrapping can be used to increase the voltage swing of a transistor amplifier, thereby improving its gain and linearity.
Switching circuits: Bootstrapping can be used in switch mode power supplies to drive high-side MOSFETs, which would otherwise require a higher voltage source.
Sampling circuits: Bootstrapping can be used to improve the accuracy of analog-to-digital converters by increasing the input impedance of the converter and reducing the loading effect on the input signal.
Active filters: Bootstrapping can be used to implement high-pass and low-pass filters with high impedance inputs, allowing the filter to have a wider bandwidth and improved performance.
Self-biasing circuits: Bootstrapping can be used to implement self-biasing circuits, which eliminate the need for external bias voltage sources and reduce circuit complexity.
These are just a few of the many applications of bootstrapping in electronic circuits. Overall, bootstrapping is a versatile technique that provides many benefits in electronic circuit design and has a wide range of applications in various fields.
