- Amplifier Gain Definition: Amplifier gain is the measure of an amplifier’s ability to increase the magnitude of a signal, expressed as a ratio without units.
- Types of Gain: Amplifier gain can be current gain, voltage gain, or power gain, each comparing different output and input parameters.
- Decibel Gain Definition: Decibel gain is defined as 10 times the logarithm (base 10) of the power ratio between the amplifier’s output and input.
- Advantages of Decibel Scale: The decibel scale makes it easier to handle large variations in gain and simplifies calculations in cascaded amplifier systems.
- Human Hearing and Decibels: The human ear perceives sound logarithmically, making the decibel scale ideal for representing sound intensity gains.
An amplifier is an electronic device that uses a small electrical signal to control a larger one. The output signal can be controlled by voltage or current. An amplifier creates a larger copy of the input signal using an external DC source. Therefore, every amplifier needs an external power source. Each amplifier has a specific ability to amplify a signal, known as gain. Gain is a ratio, so it has no units.
The ratio of output current to input current of an amplifier is defined as current gain, the ratio of output voltage to input voltage is defined as voltage gain and similarly the ratio of output power to input power of an amplifier is known as power gain of amplifier. For example if the input voltage of an amplifier is 2.5 VRMS and output voltage is 50 VRMS, the voltage gain would be,
An amplifier does not always amplify the input signal to the same extent. It may respond differently to AC and DC signals. Another way to represent gain is by the ratio of the change in the output signal to the change in the input signal. This shows how much the output changes for a given input change. While you can calculate gain using the rms value of output and input signals, this method is not suitable for AC signals.
Decibel Gain or dB Gain
Amplifier gain has no units since it’s a ratio of similar signals. However, when represented on a logarithmic scale, it has a unit called the decibel (dB), which is one-tenth of a bel. The term “bel” comes from Alexander Graham Bell, the inventor known for the telephone.
Now, decibel gain is defined in terms of the common (base 10) logarithm of a power ratio that means if the conventional gain of an amplifier is Poutput / Pinput, then this gain is expressed in decibel scale as 10log(Poutput / Pinput).
If we show the table below, it will be more clearly understood.
| Gain as ratio | Gain as decibel |
| 1000 | 30 dB |
| 100 | 20 dB |
| 10 | 10 dB |
| 1 | 0 dB |
| 0.1 | – 10 dB |
| 0.01 | – 20 dB |
| 0.001 | – 30 dB |
| 0.0001 | – 40 dB |
Why should we use Bel Scale or Decibel Scale to represent Gain?
Human hearing is logarithmic, meaning to double the perceived intensity of sound, the sound power must be increased by 10 times. Thus, an amplifier needs a gain of 10 to double sound intensity, equivalent to 1 bel or 10 decibels (dB). Similarly, the Richter scale for earthquakes and pH scale for hydrogen ion concentration use logarithmic measurements. Logarithmic scales are preferred for large variations in gain, as they offer a wide range of expression with small numerical values. In cascaded amplifiers, overall dB gain is found by adding individual dB gains, simplifying calculations compared to multiplying ratio gains.
Suppose one amplifier has gain of 3 and another has 5 and these two amplifiers are connected in cascaded manner. Overall gain of the system is 3 × 5 = 15. Now in logarithmic or dB scale these gain are 10 log3 = 4.77 dB and 10 log 5 = 6.99 dB respectively. In decibel scale this overall gain is 10 log15 = 11.76 dB which is nothing but 4.77 + 6.99. Thus we have seen how overall gain can be calculated by adding individual gain instead of multiplying them.
