Why Amplifier Has High Input and Low Output Impedance?

Amplifiers are electric components used in electric circuits to increase the gain of specific signals. The signals are passed from a source to the amplifier, then amplified and left to reach the output component, and this concept is known in many devices like voltage dividers.

Why Amplifier Has High Input Impedance and Low Output Impedance?

The amplifier has high input impedance and low output impedance due to the voltage divider’s concept, which is how the voltage in a circuit is divided depending on the impedance amount present in given parts of a circuit. An amplifier is a voltage gain device that amplifies the voltage fed to them.

The amount of voltage leaving the amplifier has the same signal as the amount of voltage entering the amplifier, but it has a more significant gain. The voltage across must be dropped for the amplifier receives the voltage signal as its input. The concept of the voltage divider states that the voltage is dropped across the components with high impedance.

This happens proportionally according to Ohm’s law through the formula V=IR; as a result, the greater the impedance (resistance) of a component, the greater the voltage drop across the component. Therefore, it must have a very high input impedance to make the voltage altogether drop across the amplifier.

If the amplifier’s input impedance is low, then the voltage will not drop entirely across it, resulting in the amplifier not receiving the signal. When it comes to the output impedance of the amplifier, it follows the same concept as the voltage divider.

After the voltage is dropped across the amplifier and the signal has been amplified, the signal or the voltage must be dropped in the device the amplifying is feeding. For example, if you have a microphone circuit, the signals that need to be amplified are the signals that are provided into the microphone.

The signals will enter the amplifier because it has a very high impedance, and the amplifier will amplify it. Then, the signals will be transmitted to the speakers because the amplifier’s output impedance is very low. If the amplifier impedance is not low, the signals will be dropped across it and only part of the speakers.

This will result in the speakers receiving no or very low signals, making the speakers play no sounds. This is the main reason an amplifier must have a very high impedance and a very low impedance.

Does Amplifier’s High Input Impedance Protect the Circuit?

The high-input impedance is essential because of the loading effect; if the amplifier’s input impedance is very low, it will draw vast amounts of current into it. Thus, this will result in a high load on the circuit, which will have hazardous consequences.

The input impedance of the amplifier must be very high to ensure that the amplifier draws a very tiny amount of the current and will not result in a loading effect on the circuit. The circuit needs a high amount of current, so the amplifier shouldn’t consume the more significant part of the current.

The high input impedance of the amplifier serves as a minimal load on the circuit because Ohm’s law states the more significant the resistance of a component, the smaller the amount of current it draws. Circuits with low impedance demand very high currents, which can be very dangerous.

The amplifier avoids this because it has a very high input impedance, which makes the input and outputs current very low. As a result, the amplifier is a low-current, high-voltage gain device. If the amplifier was a low input impedance device, a large amount of current would pass from the power source to the amplifier.

As a result of passing the current from the source to the amplifier will make the signal vulnerable to the wire or the cable’s characteristics connecting the source and the amplifier. Thus, if the wire or the cable is exposed to noise, noisy, or produces noise, the signal entering the amplifier will be noisy, and the signal leaving the amplifier will be much nosier. 


What Is the Ideal Impedance of an Amplifier?

The ideal impedance of an amplifier is infinite impedance, which means that there will be no current into or out of the non-inverting or inverting input terminals as the current flowing into the leads will be zero. If the output impedance is zero, the output voltage will be independent of the output current. 

As a result, the amplifier can output any load without the output impedance drop across it. So, the ideal input impedance of an amplifier will be infinite (high), and the perfect output impedance will be zero (low).


To summarize, the amplifier has the same concept as a voltage divider; the input impedance has to be very high to make the signals drop across the amplifier. The signals are passed from the source to the amplifier because of their high impedance, then amplified.

Meanwhile, the amplifier must have a very low output impedance to enable the signals to drop across the targeted component. If the amplifier’s input impedance is very low, it will draw massive amounts of current, resulting in a loading effect on the circuit, which can be very dangerous.

If the output impedance of the amplifier is very high, it will not allow the signals to drop across the targeted component; for example, in a microphone’s circuit, if the output impedance is very high, the speakers will receive no or very low amount of signals, producing no sounds.

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