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Current amplifier and buffers

2016-05-05 06:22  
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Current amplifier and buffers. Buffer amplifier.

Buffer amplifier is a circuit which transforms electrical impedance from one circuit to another.  The main purpose of a buffer is to prevent the loading of a preceding circuit by the succeeding one. For example, a sensor may have the capability to produce a voltage or current corresponding to a particular physical quantity it sense but it may not have the power to drive circuitry it is connected to. In such situations a buffer can be used. A buffer when connected between the sensor and the succeeding circuitry easily drives the circuitry in terms of current or voltage according to the sensor output.Buffers are classified into voltage buffers and current buffers. The symbols of ideal voltage buffer and current buffer are shown in Fig 1 and Fig 2 respectively.

ideal voltage buffer symbol

Ideal voltage buffer symbol

ideal current buffer symbol

Ideal current buffer symbol

Voltage buffer.

A circuit which transfers a voltage from a circuit with high output impedance to a circuit with low input impedance is call a voltage buffer. The voltage buffer connected between these two circuit prevents the low input impedance circuit ( second one) from loading the first one. Infinite input impedance, zero output impedance, absolute linearity, high speed etc are the features on an ideal voltage buffer.

If the voltage is transferred from the first circuit to the second circuit without any change in amplitude, then such a circuit is called unity gain voltage buffer or voltage follower. The output voltage just tracks or follows the input voltage. The voltage gain of the voltage follower is unity (Av = 1). Even though there is no voltage gain, there will be a sufficient amount of gain in current. So when a voltage follower is connected between two circuit, it will transfer the voltage from first one to second one without any change in amplitude and drives the second circuit without loading the first circuit.

A voltage buffer can be realized using opamp, BJT or MOSFET. Voltage follower using transistor (BJT) is shown in Fig 3. Voltage follower using BJT is also known as emitter follower. +Vcc is the transistor’s collector voltage, Vin is the input voltage, Vout is the output voltage and Re is the transistors emitter resistor.

Voltage follower implemented using opamp is shown in Fig 2. This is done by applying full series negative feedback to the opamp ie; by connecting the output pin to the inverting input pin. Here the opamp is configured in non inverting mode (refer Figure 2). So the equation for gain is Av= 1 + (Rf/R1).

Since output and inverting input are shorted ,Rf=0 .

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

There fore (Rf/R1) = (0/∞)  = 0.

Therefore Voltage gain Av = 1 + (Rf/R1) = 1+0 =1.

voltage buffer using transistor

Voltage follower using transistor

voltage buffer using opamp

Voltage follower using opamp

Current buffer.

Current buffer is a circuit that is used to transfer current from a low input impedance circuit to a circuit having high input impedance. The current buffer circuit connected in between the two circuits prevents the second circuit from loading the first circuit. The features of an ideal current buffer are infinite input impedance, zero output impedance, high linearity and fast response. A current buffer with unity gain (B=1) is called a unity gain current buffer or current follower. Here the output current just tracks or follows the input current. A current buffer can be realised using transistor (BJT or MOSFET).

Current amplifier circuit.

A current amplifier circuit is a circuit which amplifies the input current by a fixed factor and feeds it to the succeeding circuit. A current amplifier is somewhat similar to a voltage buffer but the difference is that an ideal voltage buffer will try to deliver whatever current required by the load while keeping the input and output voltages same, where a current amplifier supplies the succeeding stage with a current that is a fixed multiple of the input current. A current amplifier can be realized using transistors.The schematic of a current amplifier circuit using transistors is shown in the figure below. Two transistors are used in this circuit.  β1 and  β2 are the current gains of transistors Q1 and Q2 respectively. Iin is the input current, Iout is the output current and+Vcc is the transistor T2’s collector voltage  The equation for the output current is Iout = β1 β2 Iin .

transistor current amplifier

Current amplifier using transistors