Wednesday, February 4, 2009

Non-inverting Amplifier

Non-inverting Amplifier

The second basic configuration of an operational amplifier circuit is that of a Non-inverting Amplifier. In this configuration, the input voltage signal, (Vin) is applied directly to the Non-inverting (+) input terminal which means that the output gain of the amplifier becomes "Positive" in value in contrast to the "Inverting Amplifier" circuit we saw in the last tutorial and whose output gain is negative in value. Feedback control of the non-inverting amplifier is achieved by applying a small part of the output voltage signal back to the inverting (-) input terminal via a Rf - R2 voltage divider network, again producing negative feedback.

This produces a Non-inverting Amplifier circuit with very good stability, a very high input impedance, Rin approaching infinity (as no current flows into the positive input terminal) and a low output impedance, Rout as shown below.

Non-inverting Amplifier

Non-inverting Operational Amplifier

In the previous Inverting Amplifier tutorial, we said that "no current flows into the input" of the amplifier and that "V1 equals V2". This was because the junction of the input and feedback signal (V1) are at the same potential in other words the junction is a "Virtual Earth" summing point. Because of this virtual earth node the resistors, Rf and R2 form a simple voltage divider network across the amplifier and the voltage gain of the circuit is determined by the ratios of R2 and Rf as shown below.

Equivalent Voltage Divider Network

Non-inverting Amplifier Voltage Divider Network

Then using the formula to calculate the output voltage of a potential divider network, we can calculate the output Voltage Gain of the Non-inverting Amplifier as:

Non-inverting Amplifier Gain Calculation

Then the closed loop voltage gain of a Non-inverting Amplifier is given as:

Non-inverting Amplifier Gain

We can see that the overall gain of a Non-Inverting Amplifier is greater but never less than 1 (unity), is positive and is determined by the ratio of the values of Rf and R2. If the feedback resistor Rf is zero the gain will be equal to 1 (unity), and if resistor R2 is zero the gain will approach infinity, but in practice it will be limited to the operational amplifiers open-loop differential gain, (Ao).

Voltage Follower (Unity Gain Buffer)

If we made the feedback resistor, Rf = 0 then the circuit will have a fixed gain of "1" and would be classed as a Voltage Follower and this type of Non-inverting amplifier circuit is sometimes called a Voltage follower with gain. As the input signal is connected directly to the non-inverting input of the amplifier the output signal is not inverted resulting in the output voltage being equal to the input voltage, Vout = Vin. This then makes the Voltage Follower circuit ideal as a Unity Gain Buffer circuit because of its isolation properties as impedance or circuit isolation is more important than amplification. The input impedance of the voltage follower circuit is very high, typically above 1MΩ as it is equal to that of the operational amplifiers input, since an ideal op-amp condition is assumed.

Voltage Follower Circuit

In this circuit, Rin has increased to infinity and Rf reduced to zero, the feedback is 100% and Vout is exactly equal to Vin giving it a fixed gain of 1 or unity. As the input voltage Vin is applied to the non-inverting input the gain of the amplifier is given as:

Unity Gain Buffer

Since no current flows into the non-inverting input terminal the input impedance is infinite and also no current flows through the feedback loop so any value of resistance may be placed in the feedback loop without affecting the characteristics of the circuit as no voltage is dissipated across it, zero current, zero voltage, zero power drop. However in most real unity gain buffer circuits a low value (typically 1kΩ) resistor is required to reduce any offset input leakage currents, and also if the operational amplifier is of a current feedback type.

The voltage follower or unity gain buffer is a special and very useful type of Non-inverting amplifier circuit that is commonly used in electronics to isolated circuits from each other especially in High-order state variable or Sallen-Key type active filters to separate one filter stage from the other. Typical digital buffer IC's available are the 74LS125 Quad 3-state buffer or the more common 74LS244 Octal buffer.

One final thought, the output voltage gain of the voltage follower circuit with closed loop gain is Unity, the voltage gain of an ideal operational amplifier with open loop gain (no feedback) is Infinite. Then by carefully selecting the feedback components we can control the amount of gain produced by an Operational Amplifier anywhere from 1 to infinity.

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