Current amplifiers and buffers

buffer amplifier

Buffer amplifiers convert electrical impedance from one circuit to another. The main purpose of a buffer is to prevent subsequent loading of preceding circuits. For example, a sensor may have the ability to generate a voltage or current that corresponds to a specific physical quantity it senses, but it may not have a driver circuit that is connected to a power source. In this case, a buffer can be used. A buffer is connected between a sensor and a subsequent circuit when it is easy to drive the circuit's current or voltage according to the sensor output.Buffers are classified as voltage buffers and current buffers. The symbols for the ideal voltage buffer and the current buffer are shown in Figures 1 and 2 respectively.

ideal voltage buffer symbol

Ideal current buffer symbol

voltage buffer

A voltage buffer is called a voltage buffer in a circuit that transmits a voltage to a circuit with a low input impedance and a high output impedance. The connection between these two circuits is buffered by a voltage buffer that prevents the first from loading the low input impedance circuit (the second). Infinite input impedance, zero output impedance, absolute linearity, high speed, etc. are the functions of an ideal voltage buffer.

If the voltage is transferred from the first circuit to the second circuit without any change in amplitude, such a circuit is called a unity gain voltage buffer or voltage follower. The output voltage tracks or follows the input voltage. The voltage gain of the voltage follower is unity (AV = 1). Even if there is no voltage gain, there will be a sufficient amount of current gain. Therefore, when the voltage follows the connection between two circuits, it will deliver the voltage without any change in amplitude from the first to the second, with the second circuit driving without loading the first circuit.

This is possible using voltage buffering of op amps, BJTs or MOSFETs. The voltage follower uses a transistor (BJT) as shown in Figure 3. Voltage followers also known as emitter followers use BJTs. + VCC is the collector voltage of the transistor, Vin is the input voltage, Vout is the output voltage and then is the transistor emitter resistance.

A voltage follower implemented using an operational amplifier is shown in Figure 2. This is achieved by applying full range negative feedback to the op amp, i.e. the output pin is connected to the inverting input pin. The op amp here is configured in non-inverting mode (see Figure 2). Therefore, the gain equation is = 1 + (Rf/R1).

Since the output and inverting input are short-circuited, RF = 0.

Since there is no R1 to ground, it can be considered an open circuit, so R1 =∞

Therefore (Rf/R1)=(0 /∞)= 0.

Therefore, voltage gain AV = 1 +(Rf/R1)= 1 +0 = 1.

Voltage follower using transistor

Voltage follower using op amp

current buffer

Current buffers are circuits used to transfer current from a low input impedance circuit to a high input impedance circuit. The current buffer circuit in the connection between the two circuits prevents the second circuit from loading the first circuit. An ideal current buffer features infinite input impedance, zero output impedance, high linearity and fast response. A unity gain (B = 1) current buffer is called a unity gain current buffer or current follower. Here the output current tracks or is below the input current. Using a transistor (BJT or MOSFET), a buffer of current can be implemented.

Current amplifier circuit

The current amplifier circuit is an amplification circuit by a fixed factor in which the input current is fed to the subsequent circuit. Current-voltage buffer amplifiers are somewhat similar, but the difference is that an ideal voltage buffer will try to supply whatever current is required through the load while maintaining the same input and output voltages, and the current amplifier supplies subsequent stages with the current input current being fixed. multiple. Current amplifier can be implemented using transistors.The circuit principle of current amplifier using transistors is shown in the figure below. Two transistors are used in this circuit. β1 and β2 transistors Q1 and Q2 are the current profit and loss respectively. IIN is the input current, the output current is the output current and the calculation formula of the output current is the output current, VCC is the collector voltage of transistor T2 = β1β2IIN.