Example Data Table
| Case |
Ic mA |
RC ohm |
RL ohm |
RS ohm |
Use |
| Low source RF stage |
2 |
2200 |
4700 |
50 |
Wideband voltage gain |
| Small signal buffer |
1 |
4700 |
10000 |
100 |
Impedance matching |
| Higher current stage |
5 |
1000 |
2000 |
25 |
Low input resistance |
Formula Used
The calculator uses the small signal transistor model. Thermal voltage is entered by the user. Room temperature designs often use about 25.85 mV.
gm = IC / VT
re = 1 / gm
α = β / (β + 1)
rπ = β / gm
Rin ≈ ((rπ + RB) / (β + 1)) + RE
Rout ≈ RC || ro, where ro = VA / IC.
Av ≈ α × (RC || RL || ro) / Rin
Av from source ≈ Av × Rin / (Rin + RS)
fc = 1 / (2πRC) for coupling, bypass, and stray-capacitance corner estimates.
How to Use This Calculator
Enter the collector current, transistor beta, and thermal voltage first. Then add collector resistance, load resistance, and source resistance.
Use emitter resistance when part of the emitter path is not bypassed. Enter zero when no extra degeneration is used.
Add Early voltage when you want output resistance included. Use zero when you want to ignore transistor output resistance.
Enter coupling capacitors and stray capacitances to estimate lower and upper frequency corners. Press the calculate button. The result appears above the form.
Use CSV for spreadsheet checks. Use PDF for reports, notes, and design records.
Understanding Common Base Amplifiers
A common base transistor amplifier is useful when a signal source has low resistance. The base is held at AC ground. The input enters the emitter. The output is taken from the collector. This layout gives low input resistance and high output resistance.
Why This Calculator Helps
Manual design can be slow. Small changes in collector current, load, or source resistance can shift gain fast. This calculator estimates the main small signal values in one place. It finds transconductance, dynamic emitter resistance, voltage gain, current gain, power gain, input resistance, output resistance, and capacitor corner frequencies. It also shows the loaded gain, so the answer reflects the connected circuit.
Important Design Ideas
The small signal model uses the thermal voltage. At room temperature, many designs use about 25.85 millivolts. Collector current divided by thermal voltage gives transconductance. The inverse of transconductance gives emitter resistance. A larger collector current lowers emitter resistance. That can raise voltage gain, but it also raises power and bias stress.
The common base stage is often selected for radio circuits, wideband buffers, and impedance matching. It can pass high frequency signals well because the input does not see the same Miller effect found in a common emitter stage. It also has no phase inversion between emitter input and collector output in the simplified model.
Reading The Results
Voltage gain depends on collector resistance, external load, and emitter resistance. Source resistance reduces the actual signal entering the emitter. The calculator includes this loading effect. Input resistance helps you check whether the source can drive the amplifier. Power gain combines voltage gain and current gain. The frequency estimates use coupling and bypass capacitors with the resistance they see.
Practical Notes
These equations are design estimates. Real circuits include transistor capacitances, layout parasitics, temperature drift, and bias network loading. Use the result as a strong starting point. Then verify the design with measurements or circuit simulation. Keep transistor power within safe limits. Also choose capacitors with enough voltage rating and tolerance for the application.
When to Recheck
Recheck the numbers when temperature, transistor type, supply voltage, or load changes. The stage is sensitive to bias. A review prevents weak gain, clipping, and unwanted signal loss.
FAQs
What is a common base amplifier?
It is a transistor amplifier where the base is held at AC ground. The signal enters the emitter, and the output is taken from the collector. It has low input resistance and useful high frequency behavior.
Does a common base amplifier invert phase?
No. In the simple small signal model, the output voltage is in phase with the emitter input signal. This differs from a common emitter amplifier, which usually inverts the signal.
Why is input resistance low?
The emitter input sees the small dynamic emitter resistance. This value is often only a few ohms to a few dozen ohms. It depends strongly on collector current.
What is transconductance?
Transconductance shows how much collector current changes for a small input voltage change. It is calculated as collector current divided by thermal voltage. Higher collector current gives higher transconductance.
What does Early voltage do here?
Early voltage estimates transistor output resistance. A larger Early voltage gives higher output resistance. Enter zero if you want to ignore this effect in a quick estimate.
Why is source resistance included?
Source resistance forms a divider with amplifier input resistance. This reduces the signal that actually reaches the emitter. Including it gives a more realistic gain from the source.
Are the bandwidth values exact?
No. They are first order estimates. Real bandwidth also depends on transistor capacitances, wiring, layout, device package, bias network, and load behavior.
Can this replace circuit simulation?
No. It is best for design planning and checking. Use simulation or measurement before final hardware work, especially for radio, wideband, or high power circuits.