Analysis and design of AC amplifier circuit composed of integrated operational amplifier

Analysis and design of AC amplifier circuit composed of integrated operational amplifier

0 Preface

Integrated operational amplifiers (referred to as integrated operational amplifiers or operational amplifiers) are widely used in electronic circuits. Most typical application circuits of operational amplifiers have been analyzed in detail and in-depth in various electronic technology textbooks. However, many textbooks do not introduce the use of integrated operational amplifiers to form AC signal amplification circuits. Although some textbooks introduce it, the introduction is simple and the analysis is difficult. comprehensive. AC amplifier circuits composed of integrated operational amplifiers have the advantages of simple circuits, no need for debugging, and low failure rates. Nowadays, AC amplifier circuits in many electronic products are generally composed of operational amplifiers. A comprehensive analysis of the various AC amplifier circuits composed of integrated operational amplifiers. The composition and parameter calculation are helpful for the maintenance of this type of circuit, as well as the reasonable design and use of AC amplifier circuits composed of integrated operational amplifiers.

1 Analysis of operational amplifier AC amplifier circuit

1.1 Operational amplifier AC amplifier circuit using dual power supplies

In order to make the op amp have zero output when there is zero input, the internal circuit of the op amp is designed according to the requirements of using dual power supplies. The operational amplifier AC amplifier circuit uses dual power supplies to increase the dynamic range.

1.1.1 Dual power supply non-inverting input AC amplifier circuit

Figure 1 is a non-inverting input AC amplifier circuit using dual power supplies. The two sets of power supply voltages VCC and VEE are equal. C1 and C2 are input and output coupling capacitors; R1 forms a DC path at the non-inverting input end of the op amp, and the internal differential tube obtains the necessary input bias current; RF introduces DC and AC negative feedback, and makes the inverting input end of the integrated op amp A DC path is formed, and the internal differential tube obtains the necessary input bias current; since C blocks DC, DC forms full feedback, and AC is shunted through R and C, forming AC partial feedback, which is voltage series negative feedback. After the introduction of DC full feedback and AC partial feedback, the DC voltage gain can still be small (1 times) when the AC voltage gain is large, thereby avoiding saturation of the operational amplifier caused by the input offset current.

When there is no signal input, the voltage V0 at the output terminal of the op amp is ≈ 0V, and the output voltage U0 of the AC amplifier circuit is 0V; when the AC signal is input, the voltage V0 at the output terminal of the op amp changes between -VEE ~ +VCC, and is amplified by the C2 output AC signal, the amplitude of the output voltage uo is approximately VCC (VCC=VEE). After introducing deep voltage series negative feedback, the voltage gain of the amplifier circuit

For the amplifier circuit input resistance Ri=R1//γif. γif is the closed-loop input resistance of the op amp after introducing series negative feedback. γif is very large, so Ri=R1/γif≈R1; the output resistance of the amplifier circuit R0=γof≈0, γof is the closed-loop output resistance after the op amp introduces voltage negative feedback, and rof is very small.

1.1.2 Dual power supply inverting input AC amplifier circuit

Figure 2 is an inverting input AC amplifier circuit using dual power supplies. The two sets of power supply voltages VCC and VEE are equal. RF introduces DC and AC negative feedback, and C1 blocks DC to form full feedback on DC. AC is shunted through R and C1 to form partial AC feedback, which is voltage parallel negative feedback. In order to reduce the zero point drift caused by the input bias current of the op amp, R1=RF can be selected. After introducing deep voltage parallel negative feedback, the voltage gain of the amplifier circuit is

Because the inverting input terminal of the op amp is "virtual ground", the input resistance of the amplifier circuit Ri≈R; the output resistance of the amplifier circuit R0=r0f≈0.

1.2 Op amp AC amplifier circuit using single power supply

In an AC amplifier circuit using capacitive coupling, when the DC voltage at the output end of the integrated operational amplifier is not zero in the static state, the output voltage of the amplifier circuit is still zero due to the DC blocking effect of the output coupling capacitor. Therefore, there is no need to integrate an operational amplifier to meet the requirement of zero output at zero input. Therefore, the integrated operational amplifier can be powered by a single power supply. Its -VEE terminal is connected to "ground" (that is, the negative pole of the DC power supply), and the +Vcc terminal of the integrated operational amplifier is connected to the positive pole of the DC power supply. At this time, the voltage V0 at the output terminal of the operational amplifier can only be Changes between 0～+Vcc. In an op amp AC amplifier circuit powered by a single power supply, in order to prevent the amplified AC signal from being distorted, the voltage V0 at the output end of the op amp generally needs to be set between 0 and +Vcc in the static state, that is, V0 = +Vcc /2. In this way, a larger dynamic range can be obtained; when dynamically, V0 increases to a value close to +Vcc based on the value of +Vcc/2, and drops to close to 0V, and the amplitude of the output voltage uo is approximately Vcc/2.

1.2.1 Single power supply non-inverting input AC amplifier circuit

Figure 3 is a non-inverting input AC amplifier circuit using a single power supply. The power supply Vcc divides the voltage through R1 and R2, so that the potential of the non-inverting input terminal of the op amp is blocked from DC by C, causing RF to introduce full DC negative feedback. Therefore, the voltage at the output terminal of the op amp in static state is V0=V-≈V+=+Vcc/2; C communicates with each other, causing RF to introduce negative feedback in the AC part, which is voltage series negative feedback. The voltage gain of the amplifier circuit is

The input resistance of the amplifier circuit Ri=R1/R2/rif≈R1/R2,

The output resistance of the amplifier circuit R0=r0f≈0.

1.2.2 Single power supply inverting input AC amplifier circuit

Figure 4 is an inverting input AC amplifier circuit using a single power supply. The power supply Vcc is divided by R1 and R2, so that the potential of the non-inverting input terminal of the op amp

In order to avoid the interference of the ripple voltage of the power supply on the V+ potential, filter capacitor C3 can be connected in parallel at both ends of R2 to eliminate resonance; since C1 blocks DC, RF introduces full DC negative feedback. Therefore, when static, the voltage at the output end of the op amp is V0 = V-≈V+ = +Vcc/2; C1 communicates with each other, causing RF to introduce negative feedback in the AC part, which is voltage parallel negative feedback. The voltage gain of the amplifier circuit is

The input resistance of the amplifier circuit Ri≈R, and the output resistance of the amplifier circuit R0=r0f≈0.

2 Design of operational amplifier AC amplifier circuit

When designing a single-stage operational amplifier AC amplifier circuit,

(1) Select an integrated operational amplifier that can meet the usage requirements. In an AC amplifier circuit using capacitive coupling, the temperature drift voltage output by the AC amplifier circuit is very small because the capacitor blocks DC. Therefore, the requirements for the drift performance of integrated operational amplifiers can be reduced, and the selection of integrated operational amplifiers is mainly considered from aspects such as conversion rate, gain bandwidth, and noise. For pulse signals, wide-band AC signals and video signals, an integrated operational amplifier with a higher conversion rate and a gain bandwidth at least 10 times the maximum operating frequency should be selected. High-speed and low-noise integrated operational amplifiers are often used in audio AC amplifier circuits that require relatively high sound quality, such as dual operational amplifiers 4558 and NE5532.

(2) Determine whether to use dual power supply or single power supply. When usage conditions permit, the op amp AC amplifier circuit should use dual power supply mode as much as possible to increase the linear dynamic range. When the integrated op amp uses dual power supplies, the positive and negative power supply voltages generally need to be symmetrical. And the power supply voltage should not exceed the usage limit, and the power supply filtering should be good. In order to eliminate low-frequency self-excitation caused by the internal resistance of the power supply, 0.01 to 0.1 μF capacitors are often added between the positive and negative power supply connections and ground for decoupling. When using a single power supply, the potential of the non-inverting input terminal of the op amp should be less than the maximum common mode input voltage of the op amp.

(3) Determine whether the input signal is a non-inverting input or an inverting input. If the input resistance of the amplifier circuit is required to be relatively large, a non-inverting input AC amplifier circuit should be used. Because the increase in input resistance of the inverting input AC amplifier circuit will affect the voltage gain. It can be seen from the relevant calculation formulas in Figure 2 or Figure 4 that when the input resistance of the inverting input AC amplifier circuit is increased, the voltage gain of the circuit will decrease, and the voltage gain will also be affected by the internal resistance of the signal source. Therefore, when designing an inverting input AC amplifier circuit, it is sometimes difficult to balance the selection of input resistance and voltage gain. When using the non-phase input AC amplifier circuit in Figure 1 or Figure 3, the R1 bias resistor value in Figure 1 is appropriately increased, or the R1 and R2 voltage dividing resistor values ​​in Figure 3 are appropriately increased, the amplifier circuit can be improved. input resistance without affecting the voltage gain. In addition, in order to effectively improve the input resistance of the amplifier circuit in Figure 3, some improvements can be made to the circuit. The improved circuit is shown in Figure 5.

The amplifier circuit input resistance Ri ≈ R3. When the R3 value is large (see original selection in Figure 5), the amplifier circuit input resistance Ri value is large. Therefore, the input resistance of the amplifier circuit is significantly increased.

(4) Determine the AC amplifier circuit voltage gain. The voltage gain Au of a single-stage op amp AC amplifier circuit usually should not exceed 100 times (40dB). Excessively high voltage gain will not only reduce the passband of the amplifier circuit, but also easily induce high-frequency noise or produce self-oscillation. If you want to obtain an amplifier with a relatively large amplification factor, you can use a two-stage equal-gain operational amplifier circuit or a multi-stage equal-gain operational amplifier circuit to achieve it.

(5) Determine the resistance value in the AC amplifier circuit. In general applications, the resistance value is between 1 and 100kΩ. In high-speed applications, the resistance value is between 100Ω and 1kΩ, but it will increase power consumption. In portable designs, the resistance value is between 1 and 10M Ω, but it will increase system noise. First set the resistance value of the inverting input terminal R of the op amp in the figure, and then estimate the value of the feedback resistor RF according to the voltage gain calculation formula of the relevant circuit. It is best to use metal film resistors to reduce internal noise.

(6) Determine the capacitance value in the amplifier circuit. The size of the signal coupling capacitor determines the low-frequency characteristics of the amplifier circuit. Select the coupling capacitor value according to the frequency of the AC amplifier circuit signal. If a low-frequency AC signal, such as an audio signal, is amplified, the coupling capacitor value can be selected between 1 and 22 μF; if a high-frequency AC signal is amplified, the coupling capacitor value can be selected between 1000pF and 0.1 μF. The DC blocking capacitor value of the non-phase input AC amplifier circuit that introduces DC full feedback is estimated by C=1/20πfR. where f is the lowest frequency of the input signal. The lowest frequency of the audio signal is 20Hz. When R≥1kΩ, after the above equation estimation, when C=100 μF is selected, the requirement can be met. The filter capacitor value should be between 100 and 1000 μF.

3 Conclusion

When deep AC negative feedback is introduced, the voltage gain, input resistance, etc. of the op amp AC amplifier circuit are only related to the external resistance of the integrated op amp. Therefore, compared with the triode AC amplifier circuit, the design of the op amp AC amplifier circuit is more convenient and the consistency of the circuit parameters is also better.