Why can ordinary audio power amplifier directly listen to the sound of FM radio station in some cases? How does it amplify and detect the high-frequency electromagnetic wave propagating in space and restore the sound?

To understand this, you need a little more than the operational amplifier introduced in the university textbook   The characteristics of amplifier: OPAMP) have a deeper understanding of its operation, and on this basis, understand the concept and prevention of “electromagnetic interference rejection ratio: emirr” of operational amplifier.

This is important because the high-frequency electromagnetic interference is gradually increasing in the circuit working environment, such as high-frequency switching power supply, WiFi, Bluetooth, ZigBee and other wireless communication modules in the equipment. When designing the electronic signal conditioning circuit, if we do not prevent EMI, it is possible for the external high-frequency electromagnetic interference to invade the circuit, or even the circuit can not work.

LM386 basic characteristics

LM386 is an audio power amplifier circuit with a wide working voltage range (4 ~ 18V), providing about 500MW output power and voltage gain between 20 ~ 200.

1. LM386 internal structure

The following figure is the internal equivalent circuit diagram of LM386 produced by TI company. It includes front stage differential input, voltage amplification and power push-pull output. Since there is already a resistance negative feedback loop inside, the output stage will be automatically biased in the part under single voltage.

LM386 internal equivalent circuit diagram of TI company

The working principle of LM386 is similar to that of ordinary operational amplifier. In order to improve the stability of the circuit under deep negative feedback, there will be parasitic capacitance between the collector and base of the triode of the voltage amplification stage. When the frequency increases, the gain of the circuit will be reduced and the stability margin of the circuit working gain will be improved.

2. Frequency characteristics of LM386

In order to study the influence of high frequency signal on LM386, it is necessary to understand its frequency characteristics, that is, the amplitude gain and phase of operational amplifier change with the different spectrum of input signal. The frequency characteristics of LM386 are measured through a simple practical circuit.

The LM386 is configured as an amplifier with a gain of 200, and the input signal is coupled to the circuit from PIN3 through electrolytic capacitor 10.

LM386 experimental circuit

In the experimental circuit, a signal of about 10mV is input. When the frequency increases from 1kHz to 2MHz, the amplitude of the output signal changes with the change of frequency. The following figure also records the change of DC component of LM386 output pin. When the frequency is high, the DC component of output point also changes.

The amplitude frequency characteristics of LM386 and the output offset at different frequencies are obtained by frequency scanning

The amplitude frequency curve above shows that the LM386 is a low-pass filter. The output gain drops to the original valueThe corresponding frequency is the cut-off frequency of LM386.

The low-pass cut-off frequency of LM386 is about:

Frequency not only affects the amplitude gain of the output signal, but also causes the phase shift of the output signal. The following shows that the relationship between input and output waveforms changes with different frequencies.

The relationship between input, output waveform and input waveform of LM386 at different frequencies

By plotting the amplitude of the output signal and the phase difference between it and the input signal, the change caused by frequency can be clearly seen. With the increase of signal frequency, the amplitude of output signal decreases and the phase lags behind gradually.

Output amplitude and phase difference at different frequencies

3. Why does the high frequency signal cause the output DC bias voltage of LM386 to change?

It can be seen from the internal mechanism and basic frequency characteristics of LM386 that when the frequency of the input signal is relatively low, the voltage amplification factor of LM386 is relatively large and the phase difference between input and output is small. The internal negative feedback resistance network of IC basically balances the input signal of the input differential amplification stage with the feedback signal, After cancellation, the actual alternating signal acting on the base emitter of the input triode is relatively small. At this time, the triode works in the linear amplification state.

When the frequency of the input signal increases and exceeds the cut-off frequency (550kHz), the voltage gain of the LM386 decreases so that the feedback signal is gradually lower than the input signal. At the same time, the phase of the feedback signal gradually lags behind the input signal, which further increases the difference between the input signal and the feedback signal. Finally, the AC voltage component acting on the base emitter of the input stage triode is improved. When the AC voltage component exceeds a certain amplitude, additional rectified voltage will be generated due to the nonlinear rectification effect of the base conduction of the triode. After the voltage is amplified, it gradually affects the DC voltage of the output stage, thereby changing the DC bias of LM386.

Next, you can gradually change the amplitude of several groups of signals with different frequencies to observe the change of DC component of LM386.

Frequency in passband: 1kHz, 50KHz

Frequency of transition zone: 250kHz

Frequency in stopband: 1000khz

Influence of different frequency input signals on DC bias of LM386

1. Sinusoidal signal with frequency of 1kHz

Set the input signal as 1kHz sine wave and input LM386. The effective value amplitude of the signal gradually increases from 0.01 to 1.00v, and the corresponding output of LM386 and output DC offset change as follows:

The relationship between the increase of input signal amplitude and output signal amplitude and output DC offset

LM386 output waveform change

2. Sinusoidal signal with frequency of 50KHz

When the frequency of the input signal is 50KHz, the effective value and DC offset of the output signal change with the effective value of the input signal from 0.01V to 1.0V.

The output signal amplitude and DC offset of LM386 change with the increase of input signal amplitude at 50KHz

At 50 kHz, the output waveform changes as the effective value amplitude of the input signal increases from 0.01V to 1V

3. Sinusoidal signal with frequency of 50KHz

At 250 kHz, the relationship between the output signal and the output offset caused by the increase of the input signal

Change of LM386 output waveform at 250kHz

4. Sine wave signal with frequency of 1MHz

Relationship between LM386 output amplitude and DC offset at 1MHz

At 1MHz, the output signal of LM386 changes as the effective value of the input signal increases from 0.01 to 1.00v

5. Comparison of signal results at different frequencies

At different frequencies, the output signal rises with the increase of the amplitude of the input signal at the beginning. However, as the frequency exceeds the frequency range of LM386. The amplitude of the output signal decreases when it is higher than a certain value. The reason for the decrease can be seen from the change of the output DC component below.

At four different frequencies, the output of the op amp is the relationship between the amplitude of the input signal

The variation of DC component is shown in the figure below. For signals higher than the LM386 cut-off frequency, the output DC bias quality decreases with the increase of the amplitude of the input signal. Thus, the dynamic range of the output signal is affected, which also reduces the AC component in the output signal.

Compare the change of output DC offset caused by the increase of input signal amplitude at four frequencies

From the above experiments, it can be seen that the frequency is indeed the main reason affecting the DC offset of LM386. At the same time, the amplitude of the input signal will also affect the output DC offset.

When the effective value of the input signal is lower than 0.1V, the DC offset of LM386 changes little, which indicates that the primary rectification effect is not obvious. When the amplitude of the input signal increases, the rectification effect of the input stage increases, which drives the output DC component to decrease.

6. Experimental results of two groups of frequency sweeping

The first group: the input effective value is 0.1vrms. The following figure compares the change of output direct flow with the increase of frequency under the same input.

Change of output and DC offset corresponding to different frequencies under input 0.1vrms

Input the change of LM386 vertical offset corresponding to different frequencies under 0.1vrms

Group II: effect of input spectrum on output at 0.2vrms

Set the effective value of the input signal to 0.2V, and test the influence of the frequency of the input signal on the amplitude of the output signal and the quality of the output DC bias.

Influence of input signal spectrum on output signal and DC bias

When the input is 0.2vrms, the influence of the signal frequency on the output and offset

The DC offset of the previous two experiments changes with the increase of frequency.

It can be seen that when the amplitude of the input signal increases, the increase of frequency will make the change of DC offset more obvious.

Compare the influence of input spectrum on the DC offset of op amp under two good input voltages

The previous experimental data show that when the amplitude and frequency of the input signal increase, the former stage rectification effect of LM386 is more obvious.

If the LM386 power amplifier produced by the previous students can receive the program of local FM radio, according to the previous analysis, there are two conditions:

Condition 1: if the amplitude of the high-frequency electromagnetic wave entering the input port of LM386 is large enough, LM386 will output the rectified low-frequency signal;

Condition 2: there should also be a resonant circuit in the input circuit, and its center point is very close to the frequency of nearby FM stations. On the one hand, it will increase the amplitude of the received signal. On the other hand, it will also change the amplitude of the received FM signal by using the resonance characteristic curve, and then the LM386 of the later stage will rectify and amplify and output the corresponding modulated audio signal.

Emirr of operational amplifier

From the previous analysis, the high-frequency signal applied to the input stage of the operational amplifier will not be eliminated due to the low-pass effect of the operational amplifier. On the contrary, when the amplitude of the signal is greater than a certain degree, it will be rectified by the front stage of the operational amplifier, which will affect the DC operating point of the operational amplifier.

The input high-frequency electromagnetic interference will cause the change of output DC voltage

Although the high-frequency interference signals from the input, power supply and output of the operational amplifier will affect the output DC bias voltage, the interference from the input has the greatest impact.

The ratio of the amplitude of the input high-frequency interference signal to the change of the output DC of the operational amplifier caused by it is called the electromagnetic interference suppression ratio (emirr) of the operational amplifier.

The operational amplifier is configured as a voltage follower. The ratio of the high-frequency interference signal at the positive input to the DC change of the output of the operational amplifier caused by it is defined as emirr in +. Emirr and emirrin+


The specific calculation formula is as follows:

This numerical value will be given in the data book of the operational amplifier, which shows the ability of the operational amplifier to suppress external electromagnetic interference. If the circuit works in a bad electromagnetic environment, it is necessary to choose an operational amplifier with high emirr in the early stage of design.


If the emirr value of the operational amplifier you choose is not high? And just work in the complex environment of high-frequency interference, what should I do?


At this time, we need to make more efforts on the electromagnetic protection of the circuit system. By adding the input and output high-frequency filter circuit, effective shielding is added to the sensitive circuit area, isolation is added to the high-power part, etc. After all, no one wants his circuit to be able to listen to the broadcast content of the local FM station at any time.

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