Basics of Analog Electronics II - Amplifying Circuits
Preface: This chapter is a simple review of knowledge, suitable for knowledge review before learning hardware design, not suitable for use in exams.
1. Basic amplifier circuit
1.1 Initial circuit
如图所示：
- e s : input power
- R s : load of input power
- U o : output power
- R L : output load
- VCC: ①Provide energy for the circuit ②Provide a suitable static operating point for the circuit
- R B : Provide suitable quiescent current for the circuit
- R C : convert the amplified current signal into a voltage signal
- C 1 , C 2 : Cross-traffic
The value of each component
- The triode amplifier circuit is the amplification of small signals, and the maximum current passing through common devices is only a few hundred milliamps, so the input signal should not be too large
- For the design of the triode amplifier circuit, the value of the base resistance Rb is generally tens to hundreds of kiloohms . The collector resistance Rc is generally several thousand to tens of thousand ohms .
- C1 and C2 are used as capacitors for blocking direct traffic, and the value is generally a few microfarads to tens of microfarads
电路分析：
Total Base Voltage: U BE = U BEQ + U i
Total Base Current: i B = I BQ + i B
Total Collector Current: i C = I CQ + i C
Total U CE = VCC-i C R C
Simplification: U CE = VCC - (I CQ + i c ) R C = U CEQ + (-i C R C )
U o = -i c R C
Conclusion: output signal inverted
Multisim仿真图：
1.2 Static working point
When the circuit does not add a signal source, the DC power supply VCC provides the static value for the circuit, and the circuit works at point Q.
① When VCC is changed and R b , R c , and β remain unchanged, the Q point changes as follows:
- Raise VCC, move up and right in parallel
- Decrease VCC, move down to the left in parallel
②When changing R c and keeping VCC, R b and β unchanged, the changes are as follows:
- Increase R c , the slope of the DC load line changes, and the Q point moves closer to the saturation region
基本放大电路的不足：
Although the structure of this circuit is simple, the biggest disadvantage is that the static operating point is unstable. When the ambient temperature changes, the power supply voltage fluctuates, or the triode is replaced, the original static operating point will be changed, and the amplifier will not work in severe cases.
1.3 Voltage divider bias circuit
On the basis of the original circuit, RB2 , R e and C 3 are added .
Magnification β = R c /R e
特点：
- Voltage divider bias circuit, its static operating point,
与三极管本身的参数无关, it only depends on the VCC of the external circuit and RB1 , RB2 , R e - The emitter capacitor C 3 acts as a bypass capacitor
通交隔直to increase the amplification factor of the output stage.
Because in AC circuit analysis, if R e is introduced into the circuit, it will increase the power consumption of the power supply. A part of the output signal is consumed on R e , and now, C 1 is added , so for AC, this is completely short-circuited, so it will not affect the output.
Multisim仿真图：
2. Negative feedback amplifier circuit
定义：
Any part or all of the signal (voltage or current) at the output of the amplifying circuit is brought back to the input and superimposed with the input signal, which is called feedback.
- Negative Feedback: The incoming signal weakens the incoming signal
- Positive Feedback: The incoming signal enhances the incoming signal
A = X o /X d ,
F = X f / X o ,
A f = X o / X i = X 0 / (X d + X f ) = A / (1+AF)
A ≈ 1/F
A: Amplification factor without feedback, F: feedback factor, A f : amplification factor with feedback
X o : output volume, X f : feedback volume, X i : input volume without feedback, X d : input volume with feedback
According to the formula, it can be seen that
under the condition of deep negative feedback, that is, 1+AF>>1, at this time, the closed-loop magnification only depends on the feedback coefficient F, and has nothing to do with the open-loop magnification A.
Usually the feedback network is only composed of resistors, and the feedback coefficient F is very stable. Therefore, the closed-loop magnification must be quite stable. Factors that obviously affect the open-loop magnification, such as temperature changes, parameter changes, and power supply voltage fluctuations, will not have much impact on the closed-loop magnification.
负作用：
- Stabilize the static operating point; stabilize the magnification; improve the input resistance and output resistance, expand the passband, and improve the output signal waveform.
- After the negative feedback is introduced, the distorted signal is sent back to the input terminal through the feedback network, and superimposed with the input signal in reverse phase, and the net input signal obtained is small in the positive half cycle and large in the negative half cycle. This just makes up for the defects of the amplifier, making the output signal closer to a sine wave.
正反馈：
- When |AF=1,
the amplification amount and the feedback amount are in an inverse relationship, the circuit will generate self-excitation, and the removal of the signal source will also generate a signal output - When |AFI>1,
the amplification is too large, the output signal will continue to increase, if there is no timely restrictive measures, the signal will be severely distorted - When |AF|<1,
the amount of feedback is too large at this time, and the signal will continue to attenuate until it disappears
3. DC voltage regulator circuit
Multisim仿真图：
- The selection of the rectifier diode
(here U o is the effective value output by the secondary coil of the transformer ) should be selected so that the reverse voltage is greater than U ZMAX as the rectifier. - The selection of capacitor C1
(the voltage output through the capacitor is 1.2U O ), select the U o whose withstand voltage value is greater than 1.1 √2uo which is about 1.55 times - Selection of Transformer A transformer
whose transformer voltage is greater than the voltage of the voltage stabilizing block and whose current is greater than the required output current should be selected. - C3 is used as a bypass capacitor to filter out the interference signal from the input line and prevent self-excitation. The selected capacitor size is generally 0.3uF.
C2 and C4 are used as coupling capacitors to filter out interference on the output line, self-excited, and provide energy storage for sudden changes in the voltage on the output line. - Voltage regulator block
(1) Pins and functions of LM7805
Input: Voltage input terminal, the input voltage range is 7~35V.
Ground: Power ground terminal.
Output: Voltage output terminal, the typical value is 5V.
Maximum current: 1.5A.
(2) Pins and functions of LM7912
Ground: Power ground.
Input: Voltage input terminal, the input voltage range is -14.5~-35V.
Output: Voltage output terminal, the typical value is -12V.
Maximum current: 1.5A.
(3) LM317 pins and functions
Adjust: adjustment terminal.
Output: Voltage output terminal, which can output 1.25~37V adjustable voltage.
Input: Voltage input terminal, the maximum input voltage is 40V and
the maximum current is 1.5A.
Adjustable Voltage Regulator Design
1. During normal operation, the voltage V REF between the output terminal and the adjustment terminal is fixed at 1.25V.
2. To make the voltage regulator block work normally, it is necessary to ensure that the current between the output terminal and the adjustment terminal is greater than 3.5mA.
Therefore, if the current is 6mA, then R1 MAX =1.25V/6mA=208 ohms, therefore, R 1 can choose a 200 ohm ordinary carbon film resistor
3, if the output voltage is 12V, according to the formula V o =1.25(1+ R2/R1)V, R 2 =1720 ohms. Therefore, R 2 can choose a precision winding potentiometer of 2k ohms.
