Current Feedback Operational Amplifier

Less common than voltage feedback op amps, current feedback operational amplifiers are used in a number of designs to provide different characteristics.

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Everyone is familiar with voltage feedback amplifiers and in particular operational amplifiers. But there are also a form of amplifier or operational amplifier known as a current feedback op amp which can also be used.

These current feedback operational amplifiers are not nearly as common as their voltage cousins, but nevertheless current feedback op amps can provide useful advantages in a number of situations.

The abbreviations CFA for current feedback amplifier of CFB for current feedback are both widely used against VFA and VFB for the voltage feedback equivalents.

What is a current feedback operational amplifier

Although we are all familiar with voltage feedback operational amplifiers, those using current feedback can be a little different.As most operational amplifier circuits use feedback of some form or another for their operation, it is worth looking at current feedback op amps in this light, comparing current based op amps and current feedback operational amplifier circuits to the more familiar voltage based ones.

As the name implies, voltage feedback refers to a closed loop configuration where the error or feedback is in the form of a voltage.

Current feedback refers to any closed loop configuration where the error signal is in the form of a current. A current feedback operational amplifier responds to an error current rather than an error voltage.

The more familiar voltage feedback operational amplifier has a high input impedance and this results in zero input current (in the ideal scenario). It uses voltage feedback to maintain a zero input differential voltage.

A current feedback op amp has a low input impedance and this results in zero voltage at the inputs (in the ideal scenario) and it uses current feedback to maintain zero input differential current.

In essence the current feedback operational amplifier is a type of electronic amplifier whose the input is sensitive to current, rather than to voltage.

Internal elements of a current feedback op amp — Internal block diagram for a current feedback op amp

Like other forms of electronic component, it is possible to show the main internal elements of a current feedback operational amplifier to gain a better understanding of what is going on inside them and to provide a better view of their overall operation.

As with the normal voltage based op amps, there are two inputs. However with current feedback op amps they are rather different.

The non-inverting input is connected directly to a unity gain buffer and this gives this input a high impedance and it is also voltage driven.

In the ideal case the output impedance of the buffer with its input connected to the non-inverting input is zero, but it is denoted as R₀ in the diagram. Accordingly the error signal is a small current, i which also flows into the inverting input.

The input error current is mirrored into a high impedance T(s) and this results in a voltage of T(s) i from Ohm's Law.

The quantity T(s) is an important factor in the op amp performance specification as it represents the gain of the op amp. As it is a gain that has current at the input and voltage at the output, it is known as the transimpedance gain. It is in fact an impedance because the output is expressed in terms of voltage and the input is in current, so with output divided by the input becomes V / I and this is a resistance.

The voltage from this stage is buffered to provide the required output capability for the output as it will need to drive a load of some description and it needs the capability to do this satisfactorily.

Current feedback amplifier gain

It is worth looking at the gain of the current feedback op amp in a little more detail to see how it affects the operation of electronic circuit designs.

The gain of a typical voltage feedback amplifier is simply the output voltage divided by the input voltage. It is a simple ratio expressing the gain.

As we saw earlier for a current feedback op amp, the input is a value of current, whereas the output value is a voltage. This means that the gain is measured as a value of voltage / current, the value of gain is measured in Ohms.

As a result current feedback op amps are often called transimpedance amplifier or op amps, because the open-loop transfer function is as we saw.

However, referring to amplifiers like this as transimpedance amplifiers can result in some confusion because this term is more correctly applied to more general circuits such as current-to-voltage (I/V) converters. Obviously both current and voltage feedback op amps can be used within these electronic circuit designs. Accordingly it is wise to act with a little caution when the term transimpedance is encountered in any description. That said, the term current feedback op amp is rarely confused and is the preferred nomenclature when referring to op amp topology.

It is possible to look at how this affects the operation of a simple non-inverting operational amplifier circuit design.

It is possible to calculate the gain of the overall amplifier.

\frac{V_{out}}{V_{in}} = \frac{1 + \frac{R_{2}}{R_{1}}}{1 + \frac{R_{2}}{T(s)} (1 + \frac{R_{0}}{R_{1}} + \frac{R_{0}}{R_{2}})}

If we assume that R₀<< R₁ and R₁ <e; R₂,, then the equation can be simplified.

\frac{V_{o}}{V_{i}} = \frac{1 + \frac{R_{2}}{R_{1}}}{1 + \frac{R_{2}}{T}}

If the transimpedance gain of the operational amplifier is high - much higher than R2, then the gain equation can be simplified still further:

\frac{V_{o}}{V_{i}} = 1 + \frac{R_{2}}{R_{1}}

Note: for this to be true, then the value of R₂ cannot be too high, but for most electronic circuit designs with low values of R₂, this will hold true.

Key characteristics of a current feedback op amp

Current feedback op amps have many similarities with their more common voltage feedback relations, but there are also many differences.

It is worth summarising the key characteristics of current feedback amplifiers and in particular current feedback operational amplifiers.

Input characteristics: Current feedback op amps do not have balanced inputs like voltage feedback ones. Instead, the non-inverting input is high impedance, and the inverting input is low impedance.
Current feedback op amp gain: The gain of an amplifier is measured in the output divided by the input. For a current feedback op amp the output is measured in voltage, but as the input is current driven, this means that the current is the determining factor. Therefore the output / input is a figure in volts divided by a figure in amps and this means that the unit of the gain is Ohms - it is what s referred to as a transimpedance gain.
Non-inverting amplifier: For the non-inverting amplifier configuration. For a fixed value feedback resistor R2, the closed-loop gain of a CFB can be varied by changing R1, and this does not significantly change the closed-loop bandwidth.

Although they are more expensive than their voltage counterparts because they are less widely used, current feedback operational amplifiers have some characteristics that prove to be very useful in some circumstances.

Why use a current feedback op amp

A current feedback operational amplifier is rather different to a voltage feedback version with which most people are far more familiar.

As the more familiar and more commonly available voltage feedback op amps are cheap and widely available, there must be some advantages to using a current feedback one.

There are two main advantages to using a current feedback operational amplifier.

Gain bandwidth product: One of the primary reasons for using a current feedback amplifier, CFA is that it does not conform to the constant gain bandwidth product relationship that voltage amplifiers follow. This can be difficult to grasp at some times, because it is a factor that is built in to all voltage feedback amplifiers that are far more prevalent, so everyone is more used to this.
Slew rates: The second advantage of current feedback operational amplifiers is that they are able to achieve much higher slew rates than their voltage based counterparts.

The slew rate of any amplifier and in our case an operational amplifier is important because it determines the maximum rate at which the output can change and this can have many limitations on its bandwidth and output level etc, sometimes introducing distortion, etc.

The slew rate of an op amp of any amplifier for that matter is the maximum rate of change of the output voltage. It is measured in terms of the change in voltage in a given time - typically the units are volts per microsecond.

The superior slew rate for a current amplifier means that they are often used in circuit designs that require a high-speed, large-signal linear output. Examples of this might be DSL line drivers, arbitrary waveform generators, general audio electronic circuit designs, and many other applications.

Current feedback op amp circuits

Current feedback op amp ICs, are often produced so that they have the same pin arrangements as their voltage feedback equivalents.

Interestingly, this allows the two types to be interchanged without rewiring when the circuit design allows.

For many simple configurations, such as linear amplifiers, a current feedback op amp can be used in place of a voltage feedback one with no circuit modifications. The straightforward inverting and non-inverting amplifiers work with no modifications.

The classic four-resistor differential amplifier configuration also works with a current feedback amplifier, but the common-mode rejection ratio is inferior to that of the voltage feedback equivalent.

However some circuits including integrators, require a different circuit design to operate as they are based on the current flowing into the capacitor in the feedback loop for their operation.

Current feedback operational amplifiers are considerably less common than their voltage feedback relations. They are also more expensive, but in some circumstances, current feedback op amps can provide some useful advantages.