【Shenzhen University Electronic Competition】Bandpass filter

Requirements analysis

1. Use an operational amplifier to design a power supply circuit that converts a single power supply into a dual power supply. The external 12V input single power supply is converted into a dual power supply of plus and minus 6V and grounding to power the filter circuit in part 2.

2. Use an operational amplifier to design a band-pass filter with a voltage amplification of 5 times, a center frequency of 1kHz, and a bandwidth of 50Hz. It is powered by the dual power supply circuit designed and produced in Part 1.

3. The center frequency and amplification factor are adjustable within a certain range.

Solution design

1. The operational amplifier is a widely used component. It is a voltage amplifier with extremely high gain.

An ideal operational amplifier that works well has some characteristics

(1) Virtual short. The

so-called virtual short means that the voltages at the same direction and the reverse input terminal are nearly equal.

(2) Virtual break.

The so-called virtual break means that the current flowing into the same direction and the reverse input terminal is almost equal. is 0, that is,

(3) Voltage follower

When the output terminal of the operational amplifier and the inverting input terminal are directly connected together, according to the virtual short and virtual open characteristics mentioned in (1) (2) above, it is easy to obtain out. The advantage of this characteristic that the input and output points have equal potentials is that when we change the circuit of the output port, it will not affect the voltage division. If there is no voltage follower and are directly connected, it will be difficult to control the output with more convenient parameters. Voltage.

Therefore, the power supply circuit can be designed as a voltage follower, and then divide the voltage through resistors to lead to three ports with different potentials, which can form a bipolar power supply.

It is worth noting that only a unit-gain stable operational amplifier can directly connect the output terminal to the inverting input terminal. Otherwise, a 20-kiloohm resistor needs to be connected in series in the feedback loop. The tda2030 selected in the circuit may burn out. The solution is to change to an op amp with a small gain limit, such as 5532. The experimental principle is the same.

3. Filter part:

The filter circuit is a signal processing circuit that processes and processes the waveform and spectrum of the analog input signal, allowing the specified signal to pass while suppressing or attenuating signals in other frequency bands. Here we use an active RC filter circuit. The accuracy of the active RC filter circuit is higher than that of the passive filter, and the loss in the passband is also smaller. The filter circuits are classified according to the frequency response characteristics, including low-pass filtering, high-pass filtering. filtering, bandpass filtering, bandstop filtering and all-pass filtering. We usually analyze analog filter circuits in the complex frequency domain.

Using a multi-terminal negative feedback circuit
! [a.png] This is the core part of the entire circuit

. In addition, the op amp uses a common LM358

R3 with a resistance of 130K and a variable resistor with a series resistance of 100K. The magnification factor can be adjusted between 5-9.

The resistance of R2 is 2.2K, and a variable resistor with a resistance of 2K is connected in series. The center frequency can be adjusted between 1KHz-1.35KHz.

The reason for choosing these two parameters as variables is that changes in these two parameters have a low impact on other performances. In other words, the amplification factor and center frequency can basically realize independent adjustment

simulation and debugging

simulation. The software used for simulation is Multisim. It was found that the requirements can basically be met.

For more information, please refer to the documentation