What kind of circuit is this? Does anyone know how to analyze it?

小兵报道1

What kind of circuit is this? Does anyone know how to analyze it? [Copy link]

 

Please help me, thank you.

maychang

"What kind of circuit is this? Does anyone know how to analyze it?"

Post the circuit schematic.

小兵报道1

maychang

Xiaobing Report 1 Published on 2021-9-25 11:36

Where is the left side of R9 connected to? It is not shown in the figure.

Assuming that the left side of R9 is suspended, U1D and Q3 form an amplifier circuit, and the voltage of the left end of R14R15R16R17 to ground is approximately equal to V2. Apart from this, nothing can be seen.

PowerAnts

Broken Line (method) Waveform Transformation

小兵报道1

maychang posted on 2021-9-25 12:45 Where is the left side of R9 connected to? It is not shown in the figure. Assuming that the left side of R9 is suspended, then U1D and Q3 form an amplifier circuit, R14R15R16R17 ...

Input 0~10V at the left end of R9

maychang

Xiaobing Report 1 Published on 2021-9-25 13:00 Input 0~10V at the left end of R9

If the left end of R9 inputs 0~10V, then it is very likely as PowerAnts said on the 5th floor, this is a circuit for waveform conversion.

小兵报道1

maychang posted on 2021-9-25 18:03 If the left end of R9 inputs 0~10V, it is very likely that as PowerAnts said on the 5th floor, this is a circuit for waveform conversion.

Can you explain it in detail?

maychang

Xiaobing Report 1 Published on 2021-9-25 18:50 Can the master explain it in detail?

I can tell you in detail, but it's too late today, so I'll tell you tomorrow.

maychang

Xiaobing Report 1 Published on 2021-9-25 18:50 Can the master explain it in detail?

For a common amplifier, whether it is a common-phase amplifier circuit, an inverting amplifier circuit or an amplifier circuit with bias, its output-input characteristic curve is a straight line.

If the input voltage is placed on the horizontal axis and the output voltage is placed on the vertical axis, the output-input characteristic curve can be drawn as shown below:

maychang

Xiaobing Report 1 Published on 2021-9-25 18:50 Can the master explain it in detail?

In the above figure, curves 1 and 2 are obviously the output-input characteristic curves of the common-phase amplifier circuit, curve 3 is the output-input characteristic curve of the inverting amplifier circuit, and curve 4 is the output-input characteristic curve of the common-phase amplifier circuit with bias.

The biased common-mode amplifier circuit corresponding to curve 4 is shown in the figure below:

maychang

Xiaobing Report 1 Published on 2021-9-25 18:50 Can the master explain it in detail?

Regardless of the type of amplifier circuit, the slope of the curve indicates its amplification factor. For the common-phase amplifier circuit (curve 1 and curve 2), the slope is positive, so it is a common-phase amplifier circuit. For the inverting amplifier circuit (curve 3), the slope is negative, so it is an inverting amplifier circuit. Although curve 4 does not pass through the origin, its slope is positive, so we can say for sure: this is a common-phase amplifier circuit.

maychang

Xiaobing Report 1 Published on 2021-9-25 18:50 Can the master explain it in detail?

The difference between curve 1 and curve 2 on the 10th floor is that the slopes of their characteristic curves are different, the slope of curve 1 is smaller and the slope of curve 2 is larger. Therefore, the amplification factor of the circuit corresponding to curve 2 must be larger than that of the amplification circuit corresponding to curve 1. The slope of curve 3 is negative, and the corresponding amplification circuit is inverting amplification. The absolute value of the slope of curve 3 is between curve 1 and curve 2, so the absolute value of the amplification factor of the corresponding amplification circuit is larger than that of the amplification circuit corresponding to curve 1, but smaller than that of the amplification circuit corresponding to curve 2.

maychang

Xiaobing Report 1 Published on 2021-9-25 18:50 Can the master explain it in detail?

The output-input characteristic curves of the above amplifier circuits are all straight lines.

But in some cases, we may want the amplification factor of the amplifier circuit to be variable instead of a fixed constant. For example, we may want the output-input characteristic curve of a certain amplifier circuit to be as follows:

maychang

Xiaobing Report 1 Published on 2021-9-25 18:50 Can the master explain it in detail?

Or you may want the output-input characteristic curve to be as follows:

maychang

Xiaobing Report 1 Published on 2021-9-25 18:50 Can the master explain it in detail?

For amplifier circuits whose output-input characteristic curves are not straight lines, such as those shown in the 14th or 15th floor diagrams, one solution is to use a broken line to approximate the curve. The red broken lines in the following two figures are very close to the curve.

maychang

Xiaobing Report 1 Published on 2021-9-25 18:50 Can the master explain it in detail?

If we are going to use the broken line shown below (the turning points are marked in the figure) to approximate the output-input characteristic curve shown on the 15th floor

So what kind of circuit should we use?

maychang

Xiaobing Report 1 Published on 2021-9-25 18:50 Can the master explain it in detail?

The following circuit composed of an op amp and several diodes can realize the output-input curve of the 17th floor

maychang

Xiaobing Report 1 Published on 2021-9-25 18:50 Can the master explain it in detail?

In the circuit on the 18th floor, the inverting input of the left op amp is always at the "ground" potential (the op amp is in a "virtual short" state in the linear working state). Therefore, when the output of the op amp is between 0 and negative 0.7V, all diodes are not conducting, and the voltage gain of the op amp is determined by the ratio of the resistors R2R1. The ratio of R1R2 should obviously be determined by the slope of the section at the beginning of the 17th floor curve. Note that the slope of the 17th floor curve is positive, which is a non-inverting amplifier, while the op amp on the left of the 18th floor constitutes an inverting amplifier.

maychang

Xiaobing Report 1 Published on 2021-9-25 18:50 Can the master explain it in detail?

In the circuit on the 18th floor, when the potential of the output terminal of the left op amp is lower than negative 0.7V, the diode D1 is turned on. After the diode is turned on, the internal resistance is very small and can be regarded as zero. At this time, the feedback resistor of the left op amp is R3 and R2 in parallel, which is obviously lower than R2. It can be seen that: after D1 is turned on, the voltage amplification factor (absolute value) of the amplifier is smaller than when D1 is not turned on. The ratio (R3//R2)/R1 should conform to the slope of the second segment from the left of the curve on the 17th floor, that is, the value of R3 can be calculated from the slope of the curve on the 17th floor.