direct-coupled amplifier

direct-coupled amplifier

Direct-coupled amplifier

A device for amplifying signals with direct-current components. There are many different situations where it is necessary to amplify signals having a frequency spectrum which extends to zero. Some typical examples are amplifiers in electronic differential analyzers (analog computers), certain types of feedback control systems, medical instruments such as the electrocardiograph, and instrumentation amplifiers. Amplifiers which have capacitor coupling between stages are not usable in these cases, because the gain at zero frequency is zero. Therefore, a special form of amplifier, called a dc (direct-current) or direct-coupled amplifier, is necessary. These amplifiers will also amplify alternating-current (ac) signals. See Amplifier, Analog computer, Biomedical engineering, Control systems, Instrumentation amplifier

Some type of coupling circuit must be used between successive amplifier stages to prevent the relatively large supply voltage of one stage from appearing at the input of the following stage. These circuits must pass dc signals with the least possible amount of attenuation.

Interstage direct-coupling in transistor dc amplifiers must be implemented with special care. The use of both npn and pnp transistors is a possible solution. However, the pnp transistors available in monolithic form have relatively poor current-gain and frequency-response characteristics. If a dc amplifier is formed by a cascade of npn stages, there is a positive dc level buildup toward the positive supply voltage. This voltage buildup limits the linearity and amplitude of the available output swing. The problem can be overcome by using a level-shift stage between each stage to shift the output dc level toward the negative supply with minimum attenuation of the amplified signal. Practical dc level-shift stages suitable for monolithic circuit applications can use Zener diodes, a series of diodes, or a V_BE multiplier circuit.

It is generally recognized that the differential amplifier is the most stable dc amplifier circuit available. This is true because in this circuit the performance depends on the difference of the device parameters, and transistors can be manufactured using the planar epitaxial technique with very close matching of their parameters.

A method of amplifying dc (or slowly varying) signals by means of ac amplifiers is to modulate a carrier signal by the signal to be amplified, amplifying the modulated signal, and demodulating at the output.

The offset voltage of matched transistor pairs of differential amplifiers can be a source of serious problems in precision analog dc amplifier applications. Typically the offset voltage of matched metal oxide semiconductor (MOS) transistor pairs can be reduced to within ±20 mV by careful processing. However, even this low offset voltage in many applications is unacceptable. It is possible to reduce the effective input offset voltage to below ±1 mV by using chopper-stabilized amplifiers employing offset-nulling or auto-zero techniques. These techniques are essentially sampled-data methods and are based on the concept of measuring periodically the offset voltage and subsequently storing it as a voltage across a holding capacitor and then subtracting it from the signal plus the offset.