Current follower (LM11, LF351) circuit diagram
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Current follower (LM11, LF351) circuit diagram

Posted Date: 2024-01-21

A current follower is an amplification circuit consisting of one transistor or multiple transistors that can amplify DC or AC current signals and has high input impedance and low output impedance, so it can effectively maintain the accuracy and accuracy of the signal. stability. Its function is to amplify the input signal current to an equivalent output current. To put it simply, a current follower can be used to copy a current signal to another current loop.

What is the function of current follower

An electronic circuit whose function is to accurately copy the input current to the output terminal and keep the current at the output terminal consistent with the current at the input terminal.

The main function of the current follower is to reduce the impedance of the current to adapt to the needs of the load. When the load requires high current, the current follower can provide the corresponding current to meet the needs of the load without making it limited by impedance.

In addition, current followers can also be used in applications such as galvanic isolation and current amplification. In some special circuits, the current of the input signal may need to be amplified, and the current follower can achieve the signal amplification function by amplifying the current of the input signal.

Bias current follower (LM11, LF351) circuit diagram

As shown in the figure is a high-precision low-bias current follower circuit. The circuit uses a general-purpose integrated operational amplifier LM11C and a field-effect transistor input integrated operational amplifier LF351. The bias current of LM11C is very small (about 100pA), the offset voltage is low (about 0.6mV), and the common mode rejection ratio reaches 110dB. The LF351 op amp has an offset voltage of 10mV, a quiescent current of 3.4mA, and an input bias current of only 200PA. A2 in the circuit uses AC coupling to reduce the bias current. This circuit requires a long overload recovery time, and its common-mode input voltage cannot exceed ±l0

As shown in the figure is a high-precision low-bias current follower circuit. The circuit uses a general-purpose integrated operational amplifier LM11C and a field-effect transistor input integrated operational amplifier LF351. The bias current of LM11C is very small (about 100pA), the offset voltage is low (about 0.6mV), and the common mode rejection ratio reaches 110dB. The LF351 op amp has an offset voltage of 10mV, a quiescent current of 3.4mA, and an input bias current of only 200PA.

A2 in the circuit uses AC coupling to reduce the bias current. This circuit requires a long overload recovery time, and its common-mode input voltage cannot exceed ±10V. If A2 is changed to direct coupling, although the above shortcomings can be eliminated, its bias current will become larger when the temperature rises, causing Job instability. For a 10V step input, the circuit's output voltage can reach an accuracy level of 0.01% within 10μs.

Is the current follower a common emitter circuit or a common base circuit?

The current follower can be implemented using a common emitter circuit (Emitter Follower) or a common base circuit (Common Base). The specific choice depends on the design requirements and application.

1. Common emitter circuit (Emitter Follower): The common emitter circuit is a basic transistor amplifier circuit and can also be used as a current follower. In a common emitter circuit, the input signal is connected to the base and the output signal is taken from the emitter. The current of the output signal is almost equal to the current of the input signal, so the common emitter circuit is often used as a current follower. It can provide higher input impedance and lower output impedance, and is suitable for applications that require larger current amplification and lower output resistance.

2. Common Base: The common base circuit is another type of transistor amplifier circuit that can also be used as a current follower. In a common base circuit, the input signal is connected to the emitter and the output signal is taken from the collector. Unlike the common-emitter circuit, the voltage at the output terminal of the common-base circuit is reversed, but the current at the output terminal can still follow the current at the input terminal. Common-base circuits have higher current gain and lower input impedance, making them suitable for applications requiring higher frequency response and larger current gain.

Review Editor: Huang Fei

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