This is a simple design tool for calculating bias resistor values,
small-signal gain and input/output resistances of a common-emitter BJT amplifier.
Just 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),
but custom values can be given and those will not be overwritten.
After iterating any resistance value, always press 'Calculate results' button to update the results.
The small-signal values are evaluated in the mid-band, CB (and CE) short-circuited.
The small-signal gain is evaluated as VO/VS, and the input/output
resistances are evaluated as shown in the pic.
If CE bypass is not used, small-signal gain is about RC/RE.
|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.
|The collector resistor RC should be chosen to limit collector current to the suitable level.
RC defines also the output impedance.
|Choose the emitter resistor RE for bias stability.
It is good to have VE close to 0.6 volts.
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 4 - 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.
Use CE for RE bypass