Single-supply op amp AC amplifying circuit (including in-phase and inverting input)

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Single-Supply Op-Amp AC Amplifier Circuit

In AC amplifying circuits utilizing capacitive coupling, the output voltage of the amplifying circuit remains at zero even when the DC voltage at the output of the integrated operational amplifier is nonzero. This is because the output coupling capacitor blocks the DC component. Thus, the integrated op-amp does not need to meet the zero-output condition at zero input. Consequently, the integrated op-amp can operate on a single power supply. The -VEE terminal is grounded (i.e., the DC power supply is negative), while the +Vcc terminal is connected to the positive side of the DC power supply. In this configuration, the output voltage V0 of the operational amplifier can vary between 0 and +Vcc. To prevent distortion of the amplified AC signal, the static output voltage V0 is typically set to the midpoint of this range, i.e., V0 = +Vcc/2. This ensures a large dynamic range; during operation, V0 increases to values close to +Vcc based on the midpoint value and decreases toward 0V, resulting in an approximate output voltage amplitude of Vcc/2. Figure 3 illustrates this concept—please refer to the attached manuscript for details.

1.2.1 Single-Supply, Non-Inverting Input AC Amplifier Circuit

Figure 3 shows a non-inverting input AC amplifying circuit using a single power supply. The power supply Vcc is divided by R1 and R2, ensuring the non-inverting input terminal of the op-amp operates independently of DC, enabling full DC negative feedback through RF. Hence, the static output voltage V0 equals approximately V- ≈ V+ = +Vcc/2. AC signals pass through C, introducing negative feedback for the AC portion, which is voltage series negative feedback. The voltage gain of the amplifier circuit is calculated accordingly.

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

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

1.2.2 Single-Supply Inverting Input AC Amplifying Circuit

Figure 4 depicts an inverting input AC amplifying circuit using a single power supply. The power supply Vcc is divided by R1 and R2 to set the non-inverting input potential of the op-amp.

To avoid interference from ripple voltage affecting the V+ potential, a filter capacitor C3 can be connected in parallel with R2 to eliminate resonance. Since C1 blocks DC, RF provides full DC negative feedback. Thus, the static output voltage V0 equals V- ≈ V+ = +Vcc/2. C1 allows AC signals to pass, introducing negative feedback for the AC portion, which is voltage parallel negative feedback. The voltage gain of the amplifier circuit is determined accordingly.

The input resistance of the amplifying circuit is RiR, and the output resistance of the amplifying circuit is R0 = r0f ≈ 0.

2 Design of the Operational Amplifier AC Amplifier Circuit

When designing a single-stage op-amp AC amplifier circuit,

(1) Select an integrated operational amplifier that meets the application requirements. In AC amplifying circuits using capacitive coupling, the temperature drift voltage output from the AC amplifying circuit is small due to the DC blocking effect of the capacitors. Therefore, the requirements for the drift performance of the integrated operational amplifier can be relaxed, and the selection focuses primarily on conversion rate, gain bandwidth, and noise. For pulse signals, wideband AC signals, and video signals, an integrated op-amp with a high slew rate and a gain bandwidth of at least 10 times the maximum operating frequency should be used. High-speed and low-noise integrated operational amplifiers, such as the dual-amplifier 4558 and NE5532, are commonly used in audio AC amplifier circuits with high sound quality requirements.

(2) Determine whether to use dual power supplies or a single power supply. When conditions allow, the op-amp AC amplifier circuit should use dual power supply mode to increase the linear dynamic range. When using dual power supplies for the integrated op-amp, the positive and negative supply voltages are generally symmetrical. The power supply voltage should not exceed the usage limit, and better power supply filtering is recommended. To eliminate low-frequency self-excitation caused by the internal resistance of the power supply, a capacitor decoupling of 0.01 to 0.1 μF is often added between the positive and negative power supply lines and the ground. When using a single power supply, the non-inverting input of the op-amp must be less than the maximum common-mode input voltage of the op-amp.

(3) Determine whether the input signal is in-phase or inverting. If a large input resistance is required for the amplifier circuit, a non-inverting input AC amplifier circuit should be used. Increasing the input resistance of an inverting input AC amplifier circuit affects the voltage gain. From the calculation formulas in Figures 2 or 4, it can be seen that increasing the input resistance of the inverting input AC amplifying circuit reduces the circuit's voltage gain and is also influenced by the internal resistance of the signal source. Therefore, when designing an inverting input type AC amplifying circuit, it can be challenging to achieve both high input resistance and voltage gain simultaneously. When using the in-phase input type AC amplifying circuit shown in Figures 1 or 3, increasing the R1 bias resistor value in Figure 1 or the R1 and R2 voltage divider resistor values in Figure 3 improves the amplifier circuit's input resistance without affecting the voltage gain. Additionally, to enhance the input resistance of the amplifying circuit in Figure 3, modifications can be made to the circuit, as shown in Figure 5.

The amplifying circuit inputs the resistor Ri ≈ R3. When the R3 value is as shown in Figure 5, the input circuit resistance value of the amplifying circuit is large. Therefore, the input resistance of the amplifying circuit is significantly improved.

(4) Determine the voltage gain of the AC amplifier circuit. The voltage gain Au of a single-stage op-amp AC amplifier circuit is typically not more than 100 times (40 dB). Excessive voltage gain not only narrows the passband of the amplifier circuit but also induces high-frequency noise or self-oscillation. If a high-gain amplifier is desired, a two-stage equal-gain op-amp circuit or a multi-stage equal-gain op-amp circuit can be used.

(5) Determine the resistance values in the AC amplifying circuit. In general applications, the resistance is preferably between 1 and 100 kΩ. In high-speed applications, the resistance is between 100 Ω and 1 kΩ, but this increases power consumption. In portable designs, the resistance is between 1 and 10 MΩ, but this increases system noise. First, set the resistance value of the reverse input R of the op-amp in the diagram, and estimate the value of the feedback resistor RF according to the voltage gain calculation formula of the relevant circuit. Metal film resistors are preferred to reduce internal noise.

(6) Determine the capacitance values in the amplifying circuit. The size of the signal coupling capacitor determines the low-frequency characteristics of the amplifier circuit. The coupling capacitor value is selected based on the frequency of the AC amplifier circuit signal. If a low-frequency AC signal, such as an audio signal, is being amplified, the coupling capacitor value can be selected between 1 and 22 μF; if a high-frequency AC signal is being amplified, the coupling capacitor value can be selected between 1000 pF and 0.1 μF. The value of the DC-blocking capacitor that introduces full DC feedback into the AC input amplifier of the same phase is estimated using C = 1/20πfR. For an audio signal with a lowest frequency of 20 Hz, when R ≥ 1 kΩ, the above formula estimates that selecting C = 100 μF meets the requirements. The filter capacitor value is selected between 100 and 1000 μF.