types of amplifiers

Amplifier Types

Amplifier Summary
There are two types of commonly used amplifiers for instrumentation and test equipment: single-ended and differential. Both of these amplifiers have benefits and drawbacks in the way they are used in measurement systems.

Single-ended

• Good for measurements between any point and chassis ground
• Susceptible to noisy environments
• Same signal common reference for multiple channels
• Cannot be used for "above ground" measurements

Differential

• Amplifier the difference between two points
• Less susceptible to noisy environments (CMR)
• Can be used for "above ground" measurements up to the CMV
• Some signal common reference for multiple channels
• Possible crosstalk with wide voltage differences between channels

Isolated

• Amplifies the difference between any point and iso-common
• Commons are isolated from chassis ground, earth ground and other commons
• Less susceptible to noisy environments (IMR/CMR)
• Can be used for "above ground" measurements up to the IMV/CMV
• No crosstalk, even with wide voltage differences between channels

Single-Ended
A single-ended amplifier has only one input, and all voltages are measured in reference to signal common. In fact, single-ended is a misnomer, since the input voltage is measured relative to signal ground. With this amplifier, Vout is equal to Vin multiplied by the gain of the amplifier. This type of input is frequently used in devices such as oscilloscopes. A feature of single-ended amplifiers is that only one measurement point is needed. The following is a diagram for a single-ended amplifier:

One of the drawbacks of this amplifier type is the fact that in a multi-channel system, signal common (defined as the common point supplying power for the analog circuitry) can be common to all channels. Another disadvantage is that it is susceptible to noise (internal or external interference in the form of unpredictable voltages) on the input.

Differential
A differential amplifier has two inputs, and amplifies the difference between them. The voltage at both inputs is measured with respect to signal common.The following is a diagram for a differential amplifier:

Calculating the gain for a differential is more complex than a single-ended one. There are two gains associated with a differential amplifier, differential gain (Gd) and common gain (Gc). The output of a differential amplifier is described by the following:

Vout=[(Va-Vb) x Gd] + [Vavg x Gc] where Vavg=(Va + Vb)/2

In an ideal differential amplifier Gc would be zero, and the output of the amplifier would simply be the amplified difference between Va and Vb. Unfortunately, ideal differential amplifiers do not exist in practice, therefore Gc should be as small as possible. The ratio of the differential gain to the common gain becomes important since the goal is to make the second term in the above gain equation negligible. This is referred to as the Common Mode Rejection Ratio (CMRR) and leads to the Common Mode Rejection (CMR) specification that is usually used. The CMR specification is defined as follows:

CMR=20log(CMRR)=20log(Gd/Gc)

The goal when designing such an amplifier is to make the CMR as high as possible. A higher CMR indicates a differential amplifier that is less susceptible to voltages common to both inputs (noise). Another benefit of a high CMR is the ability to accurately measure a small voltage difference between two points that are both at a higher voltage potential. Since CMR decreases as the frequency of a signal increases, it is usually specified at a particular frequency (i.e., 60 Hz).

Another important specifications associated with differential amplifiers is Common Mode Voltage (CMR). This is defined as the maximum voltage allowed at each input with respect to signal common. It is important to realize that the CMV of an amplifier can be much higher than the measurement range of that amplifier. Generally, a higher CMV allows an amplifier to be used in a wider range of applications.

Differential amplifiers are quite common, since they do not have the advantage of single-ended amplifiers. They are useful for "above ground" measurements, as long as the CMV of the amplifier is not exceeded. They are also useful in environments where there is potential noise. One of the drawbacks of the standard differential amplifier is that in a multi-channel system, signal ground is often the same for all channels.

Isolated Inputs
An isolated input has a signal common (iso-common) that is isolated from the power supply for the analog circuitry. An additional feature is that in a multi-channel system, each channel's iso-common is independent and isolated from other channels. Isolated inputs can be either single-ended or differential, although a single-ended isolated input will have similar specifications to a differential one. An isolated differential input is useful with DC bridges, where an excitation voltage must be supplied. With isolated amplifiers, the terms Isolation Mode Voltage (IMV) and Isolation Mode Rejection (IMR) are used interchangeably with CMV and CMR. Isolated single-ended amplifiers, for example, have the same noise reducing characteristics as a non-isolated differential amplifier. The following is a diagram for an isolated single-ended amplifier:

As the diagram illustrates, the isolated common on the input of the amplifier is not referenced to the output common of the analog circuitry. In fact, the iso-common can actually be used for "above ground" measurements, up to the limitation of the CMV. In this respect, an isolated single-ended amplifier is very much like a standard differential one.

Many Astro-Med instruments make use of both transformer and optically-coupled isolated inputs, which offer such benefits as channel-to-channel isolation, noise reduction, and higher CMV in more sensitive ranges. With isolated amplifiers, the amplification stages are isolated from other circuitry. There is also no resistive path from the iso-common of any channel to chassis ground, no to any other iso-common.