Real-time small-signal analysis of voltage gain, input/output impedance, and frequency response. Compare Bode plots for Common Emitter, Common Base, and Emitter Follower configurations.
Parameters
Collector Current IC
mA
β (hFE)
RC Collector Resistor
kΩ
RL Load Resistor
kΩ
RE Emitter Resistor
kΩ
Bias Resistor RB
kΩ
Early Voltage VA
V
RE Bypass Cap
Results
—
Gain Av [dB]
—
Zin [kΩ]
—
Zout [kΩ]
—
fL [Hz]
—
fH [kHz]
—
GBW [MHz]
Bode Plot — Voltage Gain |Av| vs Frequency
Applications
CE amplifiers are used in RF front-ends and audio gain stages. CC (emitter follower) is used for impedance transformation and ADC driver buffers. CB amplifiers excel at high frequencies due to the absence of the Miller effect, useful for wideband RF matching.
What exactly is "AC analysis" for a transistor amplifier? I thought transistors just amplify a signal.
🎓
Basically, it's about how the circuit behaves for the small, fast-changing signal you want to amplify, separate from the DC power that turns the transistor on. In practice, we model the transistor with small-signal parameters like $g_m$ (transconductance). Try moving the "Collector Current IC" slider above—you'll see the gain change instantly because $g_m$ is directly proportional to IC.
🙋
Wait, really? So the three configurations (CE, CB, CC) in the simulator are just different ways to hook up the same transistor? Why do they behave so differently?
🎓
Exactly! It's all about which terminal is common to both the input and output. For instance, the Common Emitter (CE) inverts the signal and has high gain. The Common Collector (CC), or "emitter follower," doesn't amplify voltage but has a low output impedance. A common case is using CC as a buffer. Switch between the configurations in the simulator and watch the voltage gain and impedance plots transform.
🙋
That "Bypass Cap" parameter is confusing. What does it do, and why does it make the CE gain shoot up when I enable it?
🎓
Great observation! The emitter resistor RE provides DC stability but also reduces AC gain by introducing negative feedback. The bypass capacitor shorts out RE at signal frequencies, removing that feedback and restoring the full gain. In the simulator, toggle the "RE Bypass Cap" on and off—you'll see the gain jump from a modest value to a much larger one, which is a classic design trade-off between gain and stability.
Physical Model & Key Equations
The core of small-signal analysis is modeling the BJT with three key parameters derived from its DC operating point (Q-point).
$g_m$ (Transconductance): How much output current change per input voltage change (in S, or A/V). $r_\pi$: Small-signal resistance looking into the base. $r_o$: Output resistance due to the Early effect, modeling how IC varies with VCE. $V_T$ is the thermal voltage (~26 mV at room temp).
These parameters plug into the gain and impedance formulas for each configuration. The Common Emitter (CE) voltage gain is a foundational example.
The negative sign indicates signal inversion. The gain is proportional to $g_m$ and the effective load resistance, which is the parallel combination of the collector resistor ($R_C$), any external load ($R_L$), and the transistor's own output resistance ($r_o$). Changing $R_C$ or $R_L$ in the simulator directly controls this product.
Frequently Asked Questions
The main difference lies in the input and output terminals. The common emitter uses base input and collector output, providing large voltage gain with signal inversion. The common base uses emitter input and collector output, offering excellent high-frequency characteristics without signal inversion. The emitter follower uses base input and emitter output, with a voltage gain of approximately 1, high input impedance, and low output impedance.
At low frequencies, the impedance of coupling capacitors and bypass capacitors increases, causing signal attenuation and thus a drop in gain. At high frequencies, gain decreases due to the effects of transistor parasitic capacitances (Cbe, Cbc) and wiring capacitance. This tool allows you to check these cutoff frequencies in real time.
In the parameter display area at the top of the screen, the input impedance Zin and output impedance Zout for each circuit configuration are displayed numerically. Additionally, you can observe impedance changes on the frequency characteristic graph. These values are automatically updated when bias conditions or resistance values are changed.
First, check whether the DC bias point (Ic, Vce) is appropriate. If Vce is close to the saturation region or cutoff region, the small-signal model will not hold. Additionally, this tool uses an ideal small-signal model and does not account for actual transistor parasitic effects, temperature characteristics, or component tolerances. Differences from actual measurements are often due to these factors.
Real-World Applications
Audio Preamplifiers: Common Emitter (CE) stages are the workhorse for voltage gain in microphone preamps and instrument amplifiers. Designers carefully choose IC and RC to set the gain and headroom while using an emitter bypass cap to achieve the necessary amplification for weak signals.
ADC Driver Buffers: Common Collector (CC) emitter followers are perfect for driving analog-to-digital converters (ADCs). Their high input impedance doesn't load the preceding stage, and their low output impedance can quickly charge the ADC's sampling capacitor, preserving signal integrity.
RF/Wireless Front-Ends: Common Base (CB) amplifiers are frequently used in radio frequency (RF) stages, like in antenna input circuits. Their excellent high-frequency performance and lack of Miller effect make them suitable for amplifying signals in the MHz to GHz range with good stability.
Impedance Matching Networks: The CC and CB configurations are intentionally used to transform impedance levels between circuit blocks. For instance, an emitter follower (CC) can match a high-impedance sensor to a lower-impedance cable to prevent signal loss over distance.
Common Misconceptions and Points to Note
When you start using the simulator, there are a few common pitfalls. First, don't forget the prerequisites for "small-signal analysis". The calculations this tool performs fundamentally assume a proper DC bias point has been established. For example, if the collector resistor RC is too large, the bias point can enter saturation, invalidating this AC analysis. This is why, if you set RC to an extremely large value in the simulator, the voltage gain will deviate significantly from the theoretical value.
Next, understand the realistic ranges for parameters. For instance, increasing the collector current Ic from 1mA to 10mA will indeed increase the gain, but in a real circuit, heat dissipation and power consumption become significant. Also, the current gain β varies from part to part (e.g., hFE=70–700 for a 2SC1815). While the simulation uses a single value, in actual design you must always verify that the circuit will function even with the minimum specified value.
Finally, be aware of the gap between simulation and real-world measurements. The tool's calculations are based on ideal lumped-parameter models. On an actual PCB, parasitic wiring inductance and stray capacitance degrade high-frequency performance. For example, a frequency response that appears flat up to 1MHz in simulation might start rolling off at a few hundred kHz in a physical prototype. Think of simulation as the first step to understanding "behavior close to the ideal."