DC amplifiers and Basic principles.

DC amplifiers

Until recently, descriptive texts on DC amplifiers would go into great detail on their design using discrete devices. Integrated circuit technology has provided the designer with excellent amplifiers at a cost similar to an individual transistor. DC amplifiers are now regarded as a building block and it is very unusual for an engineer to design one.

Basic principles

In conventional AC amplifiers, capacitors and transformers are used to couple successive stages. This AC coupling allows the bias conditions for each stage to be totally independent. In a DC amplifier such techniques obviously cannot be used, and it is necessary to use direct coupling between stages.

Direct coupling brings problems, however. A first attempt at DC amplification could look similar to the circuit shown. Unfortunately there are several good reasons why this simple approach will not work.

Transistor characteristics vary widely, both with temperature and from device to device. Two characteristics are of particular importance in DC amplifiers: the base emitter voltage and the collector leakage current.

Base emitter voltage changes by 2 mV for every degree Celsius change in temperature. The amplifier is unable to distinguish between Vbe changes brought about by temperature, and Vbe changes brought about by the input. In most applications, the input to the amplifier is of the order of a few millivolts, and a simple amplifier could not be used if the ambient temperature changed.

Collector current leakage causes shifts in the collector voltage, which are treated as signal by successive stages. This leakage current is again temperature-dependent. The simple circuit here would thus make a reasonable thermometer, but a very poor amplifier.

The long-tail pair

The circuit arrangement below is used almost universally for DC amplifiers. Transistors TR 1 and TR2 are specially chosen to have identical characteristics and, more importantly, these characteristics vary identically with temperature. With discrete components, the two transistors would be specially selected and fastened to a heat sink to maintain temperature equality. In an integrated circuit, the characteristics match automatically, and the encapsulation ensures temperature stability.

As a result of this, changes in Vbe affect the emitter voltages identically, and as resistor R3 can be considered to supply a constant current, the collector currents and voltages are almost unaffected.

Moreover any changes that do occur, occur equally for transistors TR 1 and TR2,leaving the differential output voltage unchanged.

Changes in leakage current"affect both transistors similarly, again causing little change in the output voltage.

Suppose that the two input signals are equal in both amplitude and phase. This causes equal variations at the two collectors, and the output voltage is zero. If the input voltages are different, the collector currents are different, and the output voltage is an amplified version of the difference between the two input voltages. The circuit is therefore sometimes referred to as a differential amplifier.

The important characteristic of a differential amplifier is its ability to amplify differences between signals, but not amplify the signals themselves. Suppose:

The ability of an amplifier to amplify differential signals while rejecting common mode signals is called its common mode rejection ratio, or CMRR for short. It is defined as:

The above example has a CMRR of 250. In integrated circuits, the CMRR is so large that it is usually expressed in decibels. A 741 op­ amp, for example, has a CMRR of 90 dB.

The CMRR is improved by making resistor R3 closer to a constant current generator. This can be done by increasing its value which implies that Vee is made more negative. Usually, DC amplifiers work on a ±15 V supply, so there are practical limits as to how high resistor R3 can be made. A technique widely used is to replace resistor R3 with a transistor current source as shown. The zener diode ZD 1 defines the emitter voltage of transistor TR3, which in tum defines the collector current. With constant current sinks on the emitters of long-tail pairs, very high CMRRs can be obtained.

Chopper amplifiers

An alternative technique for DC amplification is the use of transistor or CMOS switches to chop the input to give an AC signal which can be amplified by a cheap AC amplifier. At the output, another switch restores the DC level. A block diagram is shown in (a). The two switches S1 and S2 are transistor or CMOS switches and operate alternately i.e. when switch S1 is closed S2 is open. Waveforms are given in (b). A chopper amplifier is usually followed by a filter to remove the AC component caused by the chopper switches.

Chopper amplifiers are also used where isolation is required between the input and the rest of the circuit. A typical example is found in industrial data logging systems, where plant faults could cause high voltages to appear on transducer inputs. Use of isolation amplifiers prevents damage beyond the input of the amplifiers.

Commercial isolation amplifiers are available in encapsulated form, with isolation voltages in excess of I kV. It is usually more economic for the engineer to purchase a ready-made amplifier than design his own.

Integrated circuit operational amplifier

In the following sections the applications of operational amplifiers are discussed in some detail. Before these can be discussed, though, it's necessary to describe limits and restrictions pertinent to design of the circuit.

The commonest operational amplifier is the 741, whose internal connections are shown. There are two inputs: the non-inverting (sometimes denoted by+) causes the output to move in phase with the signal; the inverting (sometimes denoted by-) causes the output to move in antiphase.

The first characteristic to be considered is the open-loop gain; for the 741 it is 200,000 (i.e. considerable). The second characteristic is the CMRR defined earlier. The data sheet gives a value of 90 dB.

To describe the next characteristics we must consider the internal workings of the amplifier.

If we connect both inputs to 0 V we might expect the output to sit at 0 V. Because of small differences in the Vbe voltages of the input transistors and the 74l's inherently large gain, however, the output voltage is certainly not zero. To bring the output to 0 V we have to move one of the inputs away from zero. The voltage necessary is called the offset voltage, denoted by V10, and is usually around 10 mV, although offsets of less than I mV are possible with high quality 74ls.

In itself the offset voltage is not particularly important, as it can be trimmed out. A very important factor is how V10 varies with temperature. This is denoted by a V10 and is usually about 5 !J.Vf'C.

The two input transistors require base current, denoted by lb. This is usually very low, around 0.5 1J.A for the 741. To minimise the effect of /b, the impedance in both input lines should be kept equal. The base current then generates equal offsets which the amplifier ignores.

Despite the close matching of transistors, the two base currents cannot, however, be equal. The inequality is denoted by /10 and is typically 0.21J.A. Unlike V10,110 does not vary greatly with temperature.

DC amplifiers are designed to work with low frequencies, and to minimise problems with stability when feedback is applied, the high frequency response of the 741 is deliberately limited by a strategi­ cally placed 30 pF capacitor on the chip. The gain falls to 60 dB at I kHz, 40 dB at 10kHz and 0 dB at I MHz. The frequency at which the gain falls to unity is called the unity gain bandwidth.

DC amplifiers without internal compensation are available, an example being the 531 operational amplifier. This has a unity gain bandwidth in excess of 10 MHz, and gives 40 dB of gain up to 500 kHz. The use of uncompensated amplifiers requires some care if instability is not to be a problem.

A second method of describing the amplifier response is the slew rate. This measures response of the amplifier to a step input. It is defined as the rate of change of the output and is measured in volts/ unit time. The slew rate for the 741 is 0.5 V/!JS.