Why input impedance of an amplifiers is high

This can be modelled as a resistor, as seen in the figure below, connected parallel to the input. The input impedance is represented in the above diagram as a resistor, since this is valid in most cases. The inputs, however, also have a tiny capacitance. At lower frequencies, this is not a concern as it just has the effect of minimizing rise and fall times. However, this capacitance can provide a substantial load for AC signals at high enough frequencies and hinder the rise and fall times, which can also contribute to distortion of the signal.

You should be able to push any amount of current through any load using an ideal amplifier. With a good output driver phase, this is possible, but in terms of how much current it can produce, a bare amplifier itself has some limitations.

As shown in the figure below, this restriction of the output drive can be considered as a resistor in series with an ideal output. Since the output is only seen on the other side of the resistor, there is a large voltage drop across the resistor if the output is overloaded, and the output is not the actual output given by the amplifier.

This can be countered by the addition of an output stage that further amplifies the signal and makes it ideal for large loads to be powered. As for an ideal op amp, an ideal op-amp has infinite input impedance. This means that there can be no current into or out of the inverting and non-inverting input terminals, because the current flow into the input leads is zero.

An ideal op-amp has zero output impedance. This means that the output voltage is independent of output current. So the ideal op amp can drive any load without an output impedance dropping voltage across it. The short summary: input impedance is "high" ideally infinite , output impedance is "low" ideally zero. Op Amp Impedance Matching. The high impedance ensures that it draws very little current. It is the amplifier's task to convert a low energy, voltage-driven signal into a higher-voltage output signal.

Low impedance circuits can be dangerous because of the high current draw that they produce. Op amps avoid this by having very high input impedance. In other words, Op amps need high input impedance because they are voltage-gain devices. It's also important to prevent the loading effect. If the impedance were small, the current draw would be high.

The variation in the input bias current is measured against the variation in the input common-mode voltage range. Input Impedance and Output Impedance of Amplifier.

On the other hand, if the load resistance is lower than the source resistance, most of the power is dissipated in the source, leading to a poor efficiency of the power transfer even if the global resistance decreases, which results in a higher magnitude of the power.

Nowadays, as a general rule, high input and low output impedances are the norm, even if it does not lead to an impedance match. However, we will see in the next section that in some cases, impedance matching can be more suitable. Basically, we can distinguish three scenarios of connection. The first one, is when a source is connected to an amplifier, this is what is shown in Figure 2.

The second case is when the amplifier is connected to a transducer. A transducer is the final stage of the circuit, it is the element that converts the electric signal into sound and movement for example, examples of transducers are loudspeakers and motors. The configuration of this connection is the same as presented in Figure 2 where the source would be the amplifier and the load the transducer.

In modern electronics, this type of architecture is very common to realize multiple operations and amplifications to the signals. In the input stage, where a power supply source R S is connected to an amplifier R L , a maximum transferred power is not necessary since the amplifier can itself re amplify the signal.

Usually, a signal loss of -6 dB between the source and the first amplifier commonly known as preamplifier is acceptable, such a loss is achieved when an impedance match is realized. In the case of the cascade configuration presented in Figure 4 , two functioning modes can be distinguished and treated differently :. For the final stage, where a last amplifier supplies a transducer lets say a loudspeaker , the output impedance of the amplifier must be lower than the internal loudspeaker resistance.

Again for the same reasons, the power is transferred more efficiently to the transducer if the amplifier has a low output impedance. In this case, most of the power can be used by the transducer. But in high speed logic, you want the input impedance to adjust the output impedance. This is especially true for RF radio signals.

TV antenna has a matching transformer to match ohm impedance from antenna to 75 ohm coax, then input of your TV has a 75 ohm impedance. Google serves cookies to analyze traffic to this site and for serving personalized ads.

Learn more. Simplified answer relating mainly to audio amplifiers: - An audio amplifier with a low output impedance can deliver larger powers to its loudspeaker more efficiently than an amplifier with higher output impedance.

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