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.
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.
|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.
|Choose the collector bias voltage to aim for.
Typically the collector bias voltage is half of the operating voltage VCC.
|Select the collector resistor RC to limit collector current to suitable level.
RC defines the output impedance.
|Choose the bootstrap resistor RBs value.
To maximise input impedance, start with 100k, for example.
|Choose the emitter resistor RE for bias stability.
The value of RE affects the base bias resistor values and input impedance.
|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 8 - 16.
|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.