This is a simple design tool for calculating bias resistor values, small-signal gain and input/output resistances of a bootstrapped common-emitter BJT amplifier. Fill the input fields below in given order from top to bottom. The ordering of the fields serves as a step-by-step guide for the design process. The base bias resistor values are calculated automatically (using 5% tolerance resistances), custom values can be given and those will not be overwritten. After iterating any resistance value, press the 'Calculate results' button to update the results. The small-signal values are evaluated in the mid-band, CB and CBs short-circuited. The small-signal gain is evaluated as VO/VS, and the input/output resistances are evaluated as shown in the pic. Small-signal gain is approximately RC/RE. The bootstrap configuration creates a high input impedance amplifier.
Select the operating voltage VCC. Typically 9 volts is the way to go. VCC: volts RESULTS

DC Bias voltages:
VC : volts
VE : volts
VB : volts

AC Voltage Gain factor:
Av : const

AC Input resistance:
Ri : ohms

AC Output resistance:
Ro : ohms

Select the BJT you want to use, from its datasheet lookup the current gain factor and place it here as a parameter. hFE: const
For convenience, you can change the base-emitter voltage, which is set to 0.65 V by default. Typical VBE is 0.55 - 0.75 V. VBE: volts
Choose the collector bias voltage to aim for. Typically the collector bias voltage is half of the operating voltage VCC. VC: volts
Select the collector resistor RC to limit collector current to suitable level. RC defines the output impedance. RC: ohms
Choose the bootstrap resistor RBs value. To maximise input impedance, start with 100k, for example. RBs: ohms
Choose the emitter resistor RE for bias stability. The value of RE affects the base bias resistor values and input impedance. RE: ohms
A multiplier for base resistor values, which are obtained in the next step. Too much resistance at the base affects the stability of biasing. Use values between 2 - 10. Bx: const
Use 'Calculate values' button to evaluate exact values for base resistors. Then use the arrow buttons to round up or down to nearest standard value, or leave as is.


Also, the internal resistance of the signal source should be given. For ideal source this value is small, but for guitar output this is a few kilohms. RS: ohms