Introduction to Amplifiers
An amplifier takes a small signal and produces a larger usable signal. The goal is not simply to make a voltage bigger; a good amplifier preserves the information in the signal while adding as little distortion, noise, loading, and delay as the application allows.
Learning Objectives
By the end of this lesson, you should be able to calculate voltage gain, express gain in decibels, explain bandwidth and slew-rate limits, compare voltage/current/power amplifiers, and choose basic amplifier properties for sensors, audio, and embedded systems.
What Gain Means
Voltage gain is the ratio between output voltage and input voltage:
[
A_V = \frac{V_{OUT}}{V_{IN}}
]
Gain in decibels is:
[
A_V(dB) = 20 \log_{10}(A_V)
]
Example: a microphone preamplifier receives 10 mV and produces 1 V.
[
A_V = \frac{1}{0.010} = 100
]
[
A_V(dB) = 20 \log_{10}(100) = 40dB
]
Try It: Amplifier Gain Calculator
Enter any two values. The calculator can solve for input voltage, output voltage, linear gain, or gain in dB.
Amplifiers Need Power
An amplifier does not create energy from nothing. It uses a DC power supply to control a larger output signal in response to a smaller input signal.
The output cannot exceed the supply rails. If the required output is beyond the available swing, the waveform clips.
Types of Amplifiers
| Type | Main goal | Example |
|---|---|---|
| Voltage amplifier | Increase voltage | Sensor preamplifier |
| Current amplifier | Increase drive current | Buffer for a load |
| Power amplifier | Deliver voltage and current | Audio speaker driver |
| Transimpedance amplifier | Convert current to voltage | Photodiode receiver |
| Instrumentation amplifier | Amplify small differential signals | Bridge sensor front-end |
Input and Output Impedance
An amplifier should usually have high input impedance so it does not load the source. It should usually have low output impedance so the next stage receives a stable signal.
For a source resistance (R_S) feeding amplifier input resistance (R_{IN}):
[
V_{AMP} = V_S \frac{R_{IN}}{R_S + R_{IN}}
]
If (R_{IN}) is not much larger than (R_S), the amplifier input attenuates the signal before amplification even begins.
Bandwidth and Gain-Bandwidth
No amplifier has infinite bandwidth. A single-pole response is down by 3 dB at its cutoff frequency. Many op-amps also have an approximate gain-bandwidth product:
[
A_{CL} f_{BW} \approx GBW
]
If an op-amp has (GBW = 1MHz), a gain of 100 leaves only about:
[
f_{BW} \approx \frac{1MHz}{100} = 10kHz
]
That may be fine for slow sensors but poor for wideband audio or fast data acquisition.
Slew Rate
Slew rate limits how fast the output can move:
[
SR \ge 2 \pi f V_{PEAK}
]
For a 20 kHz sine wave with 5 V peak:
[
SR \ge 2\pi \times 20000 \times 5 = 0.628V/us
]
Choose margin above the calculated minimum to reduce distortion.
Noise, Offset, and Distortion
Small-signal amplifiers must account for:
- input offset voltage, which appears amplified at the output;
- input bias current, which creates voltage across source resistance;
- voltage and current noise;
- harmonic distortion if the circuit is nonlinear;
- common-mode range and output swing limits.
For precision sensors, the best amplifier is often not the one with the highest gain. It is the one whose error terms are small compared with the measured signal.
Common Mistakes
- Treating gain as the only amplifier specification.
- Forgetting that output swing is limited by supply rails.
- Choosing gain without checking bandwidth.
- Ignoring input impedance and loading the sensor.
- Driving a low-impedance load directly from a weak op-amp output.
- Ignoring slew-rate distortion on large, fast signals.
Summary
Amplifiers make weak signals usable by controlling energy from a power supply. Practical designs check gain, dB, impedance, bandwidth, slew rate, noise, offset, distortion, supply rails, and load current.
Further Reading
- Texas Instruments, "Op Amps for Everyone."
- Analog Devices, "Op Amp Applications Handbook."
- Horowitz and Hill, "The Art of Electronics," amplifier chapters.