Introduction to DC Amplifier

Catalog

Ⅰ Detailed introduction

An amplifier that can amplify DC signals is called a DC amplifier. A DC amplifier amplifies a DC signal or an alternating signal that changes very slowly with time. The stages must be connected by components that can pass DC such as equal wires or resistors. Therefore, it is also called a direct coupling amplifier. There are many types of DC amplifiers. A directly coupled single-tube amplifier is the simplest one. A differential amplifier composed of a pair of transistors or field-effect transistors is a DC amplifier with a small zero drift and is often used in the input stage and intermediate stage of an integrated operational amplifier. Chopper-type DC amplifiers are also commonly used in measuring instruments.

DC amplifiers are often used in measuring instruments. In high-precision potential measurement and bioelectrical and physical electrical measurement, electrical signals are often very weak, change slowly, and contain DC components, which are easy to detect, record, and process after being amplified. In addition, in many cases, the internal resistance of the signal source under test is high, requiring the amplifier to have high gain and high input impedance. A DC amplifier with this characteristic is also suitable for use as an operational amplifier.

Both directly coupled transistor or tube amplifiers can be used as DC amplifiers. This type of amplifier also relies on a DC power supply. When there is no signal input, the output potential of an ideal DC amplifier should be zero, or a reference potential called the DC zero point of the amplifier. But in fact, due to the fluctuation of the power supply voltage and the change of environmental factors such as temperature, as well as the aging of electronic components and devices, this reference potential will change with the change of the amplifier's characteristic parameters. In this way, the output of the amplifier will inevitably contain an unfixed error called zero drift. In the case of direct coupling of multi-stage amplifiers, the zero-point drift of the previous stage will be gradually amplified by the subsequent stages, and the result will be confused with the amplified useful signal and affect the performance of the amplifier. It is an important goal in the design of a DC amplifier to overcome zero drift to the greatest extent.

There are many types of DC amplifiers. A directly coupled single-tube amplifier is the simplest one. The disadvantage of this amplifier is the large zero drift.

A dual-channel chopper-type DC amplifier consists of three parts: chopper channel, high-frequency channel, and main amplifier. The DC component (including the slow-changing component) and the high-frequency component in the measured signal are added by the main amplifier after being processed by the chopping channel and the high-frequency channel respectively. The signal through the chopping channel is "chopped" into a square wave before being amplified, and then restored to DC by the demodulator after AC amplification. AC amplifier and the low-pass filter will not produce zero-point drift. As long as the chopper is turned on and off without introducing residual voltage and leakage current, the entire amplifier will basically not produce zero-point drift. The high-frequency channel makes the components with higher signal frequency directly output through the main amplifier, which can compensate and widen the frequency band. The quality of the chopper has a great influence on the performance of the DC amplifier. Early mechanical vibrator choppers had ideal switching characteristics, but their operating frequency was only a few hundred Hz and their lifespan was short. Modern choppers mainly composed of field-effect transistors have good performance and have been widely used.

Like an operational amplifier, the chopper DC amplifier once played an important role in an analog computer, and later it was mainly used in a high-precision test system. The integrated operational amplifier can be directly used for linear DC amplification and is widely used.

Ⅱ Single-ended DC amplifier

Single-ended DC amplifier circuit diagram

Single-ended DC amplifiers need to solve the problem of inter-stage DC level configuration. Using resistor Re2 to pull down the emitter potential of BG2 to meet the DC level configuration requirements (ie Ube2=Uc1-Ue2). Use D1 and D2 for level configuration, making the bias voltage of BG2 and BG3 Ube2=0.3V and Ube3=0.45V respectively. D3 plays a protective role to avoid excessive back pressure on the base of BG1. If the main output voltage of the front stage is different from the input voltage of the rear stage, the stable voltage of the silicon zener tube can be used to replace the role of the silicon diode. The circuit in Figure C below uses larger Rc1 and Rc2 to increase the collector voltage to achieve the configuration of the front and rear DC levels. The circuit in Figure D below uses the polarity of PNP (BG1 and BG3) and NPN (BG2) to be level configuration. The output current of BG1 is the input current of BG2, and the output current of BG2 is the output current of BG2 is BG3. The input current realizes better inter-stage coupling. The biggest disadvantage of the above four circuits is the large zero drift.

Ⅲ Differential DC amplifier

A differential DC amplifier is composed of a pair of BG1 and BG2 transistors with the same characteristics, and the circuit components are also symmetrical. The input signals are Ui1 and Ui2; the single-ended output signals are Uc1 and Uc2 respectively; the double-ended output is the difference between UC1 and UC2, that is, UO=Uc1-Uc2. The differential circuit has the following characteristics:

1.Suppress zero drift

The differential circuit has the same tube characteristics and symmetrical circuit components, so when the temperature rises, the collector currents of the two tubes will get the same increment, that is, △IC1=△IC2 and the double-ended output is UO=△IC1RC-△IC2RC =0, so the output has no zero drift.

2.Suppress amplification during common mode input

Usually, a pair of input signals with the same amplitude and the same phase is called common-mode signals. It can be seen from the following circuit diagram A. When Ui1=Ui2, under symmetrical conditions, the double-ended output Uo=KUil-KUi2=0.

3.Amplifying ability in differential mode input

Usually, a pair of input signals with equal amplitude and opposite phase are called differential mode signals. When Ui1=-Ui2 differential mode input, the two-sided three-knife tube collector output is Uc1=-KUi1, Uc2=-KUi2; therefore, the differential mode magnification Kud: Kud=(Uc1-Uc2)/(Ui1-Ui2)=( -Ui1K-Ui1K)/2Ui1=-K=(-)(hfeRc)/(Rs+hie)

Since the double-ended input voltage and double-ended output voltage of the differential circuit are both doubled than the single-tube common-emitter amplifier circuit, the differential mode amplification Kud is the same as that of the single-tube common-emitter circuit.

In order to improve the ability to suppress zero drift, the smaller the common-mode amplification factor, the better, and the larger the differential-mode amplification factor, the better. Therefore, the common-mode rejection ratio CMRR*=Kud/Kuc is used as an important indicator to evaluate the performance of the differential amplifier circuit.

4.Stabilize the static operating point

The emitter resistance Re has a strong negative feedback effect on the common-mode signal and the temperature drift level. For example, when the temperature rises, IC1 and IC2 both increase at the same time, and the following negative feedback process occurs:

As a result, the actual changes of IC1 and IC2 are relatively reduced. Here Re plays a constant current role, thereby stabilizing the static operating point. Obviously, the larger Re, the greater the constant current effect, and the stronger the ability to suppress zero drift. Auxiliary electricity is used to offset the bulge of Re. The emitter can maintain a normal value to the ground potential. It is worth noting that Re does not have a negative feedback effect on the differential mode signal, so it will not reduce the amplification factor of the differential mode signal.

Ⅳ Low noise ultra-wideband DC amplifier

Low-noise wideband DC amplifiers are widely used in intermediate frequency and video amplifiers. This type of circuit is mainly used to amplify video signals, pulse signals, or radiofrequency signals. The bandwidth of the amplified signal can range from DC to several megahertz or even dozens of megahertz. Hertz is widely used in signal processing. Especially in recent years, the rapid development of Ultra-Wideband (UWB) technology in the field of covert communication and target detection has further increased the bandwidth requirements of UWB signals. Therefore, the signal preprocessing circuit required by the receiver front-end must be a low-noise ultra-wideband amplifier.

The performance of ultra-wideband DC amplifiers directly affects the accuracy of signal detection and processing. Low noise, low zero drift, and ultra-wideband design have always been the focus of this type of amplifier, which has important engineering significance and practical value. Bevilacqua et al. and VaSIC et al. designed an amplifier that can achieve ultra-wideband amplification and low noise characteristics, but they did not solve the problems of zero drift and high noise figure.

Ⅴ What is the difference between a DC amplifier and an AC amplifier?

DC amplification and AC amplification are relative, but the difference between the DC amplifier and the AC amplifier is obvious. The input and output do not use capacitors to block the DC amplifier, and the input and output capacitors are called AC amplifiers. The advantage is small distortion, and the disadvantage is that the static operating point of each stage of the circuit is related to the front and back circuits, and the output amplitude must be zero when the input signal is zero. There is no DC component, so the circuit adjustment is difficult. The advantage of the AC amplifier is that there is a capacitor to block the DC, the static operating point of each stage of the circuit has nothing to do with the front and rear stages and the circuit is easy to adjust. The disadvantage is that the frequency band of the input and output DC blocking capacitors is limited, which will always cause a certain deformation of the signal. The more cascades, the greater the distortion.