Lesson 7 Amplifier 放大器

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Electronic circuits can be divided into two groups: analog and digital.

Analog circuits use a continuous range of voltage as opposed to discrete levels as in digital circuits[1]. Good examples of analog circuits include transistor amplifiers, operational amplifiers, oscillators, filters, and power suppliers.

Amplifier is the device that increases the current, the voltage, or the power of a signal. Transistors are frequently used as amplifiers. Take a look at the common emitter transistor amplifier in figure 3-1. The input to the amplifier is a few millivolts sine wave. It is introduced into the circuit by the coupling capacitor and is applied between the base and emitter[2]. The output voltage from the amplifier, taken at the collector of Q[3], is the same sine wave voltage that increased amplitude[4], but has 180 degrees phase reversal[5].

We have built voltage and current amplifiers using transistors. Circuits of this kind with nice properties (high gain and high input impedance, for example) , packaged as integrated circuits (ICs) , are called operational amplifiers or op-amps. Op-amp name is derived by the fact that they were originally used to perform mathematical operations[6]. Today the op-amp has become an inexpensive and readily available ‘building block’ for many circuit applications.

An op-amp has very high gain, very high input impedance, and very low output impedance. An ideal op-amp has infinite gain (A=∞) , infinite input resistance (Rin =∞) , and zero output resistance (Rout= 0) . A real op-amp has a gain on the range 103-105 (depending on the type) .

Figure 3-2 shows the symbol for an op-amp. There are two inputs, the inverting input V- and the non-inverting input V+. Op-amps require two power supplies to operate, supplying a positive voltage Vs+ and a negative voltage Vs-. Though designs vary between products and manufacturers, all op-amps have basically the same internal structure. The basic architecture of the 741 is shown in figure 3-3.

Because of their very high open loop gain, op-amps are usually connected with feedback to produce a closed loop operation[7]. An op-amp circuit can be connected with an inverting amplifier configuration (Fig. 3-4) or a non-inverting amplifier configuration (Fig. 3-5) .

The voltage gain of an inverting amplifier is given as:Vout= -Vin(Rf/Rin). The voltage gain of a non-inverting amplifier is given as:.

New Words and Technical Terms

Notes to the Text

[1]Analog circuits use a continuous range of voltage as opposed to discrete levels as in digital circuits.：模拟电路使用的是连续幅值的电压，与之相反的离散值电压应用于数字电路中。

[2]It is introduced into the circuit by the coupling capacitor and is applied between the base and emitter：它（输入正弦波）通过耦合电容，施加在基极和发射极之间，引进电路。

[3]taken at the collector of Q：在晶体管的集电极取出。

[4]is the same sine wave voltage that increased amplitude：是一样的正弦波电压，电压增加了幅度。

[5]but has 180 degrees phase reversal：但是有180度相位反转。

[6]Op-amp name is derived by the fact that they were originally used to perform mathematical operations：运算放大器的名字来自于这样的事实，即它们最初是用来执行数学运算的。

[7]Op-amps are usually connected with feedback to produce a closed loop operation：运算放大器通常用反馈形式连接，产生闭环运行结构。

Exercises

Ⅰ.Choose the best technical term to complete the following statements.

1.Analog circuits include ( ), oscillators, filters, and power suppliers.

A. voltage B. amplifier C. sine wave D. feedback

2.( ) is the device that provides amplification (the increase in current, voltage, or power of a signal) without appreciably altering the original signal.

A. analog B. digital circuit C. amplifier D. power supplier

3.Most amplifiers can be classified in by their function. If the output signal is larger in voltage amplitude than the input signal, the amplifier is a ( ). If there is no voltage gain, but the output power is greater than the input power, the amplifier is a ( ).

A. voltage amplifier B. power amplifier C. transistor amplifier D. operational amplifier

4.In the common-emitter amplifier circuit, the base terminal of the transistor serves as the input, the collector is the output, and the ( ) is common to both.

A. base B. emitter C. collector D. substrate

5.In a non-inverting amplifier circuit, if the op-amp has an open-loop gain of 200000, the input voltage Vin = 1.5 volts, the inverting input resistor R1=10 kilo-ohms，the feedback resistor R2=0 ohms, then the output voltage can be calculated by using the formula. The output voltage Vout = ( ) volts.

A. 300000 B. 0 C. 1.5 D. unstable

Ⅱ.Translate the following sentences into Chinese.

1.A bipolar junction transistor consists of three regions of doped semiconductors. A small current in the center or base region can be used to control a larger current flowing between the end regions (emitter and collector) .

2.The larger collector current IC is proportional to the base current IB according to the relationship IC =βIB , or more precisely it is proportional to the base-emitter voltage VBE . The smaller base current controls the larger collector current, achieving current amplification.

3.In the emitter follower circuit the control signal is applied at the base, but the output is taken from the emitter. The emitter follower has unit gain, the emitter voltage precisely follows the base voltage. The input impedance is high and the output impedance is low.

4.In most practical applications it is better to use an op-amp as a source of gain rather than to build an amplifier from discrete transistors.

5.The operational amplifier is very cheap especially keeping in mind the fact that it contains several hundred components. The most common op-amp is the 741 and it is used in many circuits.

6.Though designs vary between products and manufacturers, all op-amps have basically the same internal structure, which consists of three stages: differential amplifier, voltage amplifier, and output amplifier.

7.Op-amp input resistance: This is the resistance with respect to circuit ground and is often many mega-ohms, especially if the input stage is based on field-effect transistors.

8.Op-amp output resistance: This is the open loop output resistance and is often in the 50 to 200 ohm range at low frequencies and typically increases with frequency.

9.Op-amp voltage gain: The voltage gain of an op-amp at DC is around 1,000,000. This gain falls as frequency rises, reaching unity at a frequency known as fT. This is known as the gainbandwidth product.

10.Common-mode voltage gain（Common-Mode Rejection Ratio）: CMRR is a measure of how well the amplifier rejects common-mode signals at the inputs. CMRR has typical specs in the 60 to 100 dB range.

11.Zero offset voltage: When both inputs are tied together, the output of the op-amp should be zero. However, due to tolerances in manufacturing, this is rarely the case. Offset voltage is the input voltage that is required to achieve an output of 0V, and is typically ranges from 0.1 to 5mV.

12.Output slew rate: Slew rate dictates how quickly an op-amp can change output voltage. Typical values range from 1 to 20V/μs for audio op-amps, much higher numbers are available for highspeed video op-amps.