Transimpedance

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Transimpedance [Copy link]

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*) , the quartz resistor is a current / voltage conversion circuit. According to Ohm's law, a resistor is enough. Conversion rate R=Rf=Vo/Ii , who doesn't understand this simple? There is indeed a voltage amplification routine, so this is not redundant. In the following simulation, whether the input Ii is a sine wave or a rectangular wave, a 1M resistor is enough. Conversion rate R=-Rf , input Ii=1uA , output Vo=1V .

captz

*) , but if the load resistor is connected in parallel to the Rf resistor, the output voltage will change accordingly, and the conversion rate R=Rf//Ro will be uncertain. The commonly used reverse input voltage follower matching isolation, the input current Ii drops IiRf at the op amp - end , the output is reversed -IiRf , the conversion rate is fixed R=-Rf , Vo=-IiRf .

The above circuit with a follower outputs Vo=-Ii*Rf=-(1.41e-6)(1e+5)=-141mV under a 1K resistor load , and does not change with the load.

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*) , Follower with same input direction

The voltage signal Vi=-1pV/100Hz is transformed and reversed into a rectangular wave of current Ii=1pA/100Hz . The voltage drop V=Ii*Rf=(1e-12)(1e+9)=1mV at the resistor Rf . The voltage follower outputs a rectangular wave of voltage Vo=1mV at the load Rz . The indispensable op amp follower parameters bring many problems. If no measures are taken, the circuit will be unstable or even stop.

wangerxian

These pictures cannot be refreshed, what are the external links to visit?

captz

captz posted on 2022-4-24 17:47 *), the voltage signal Vi=-1pV/100Hz of the follower with the same input direction is transformed and reversed into the current Ii=1pA/100 ...

*) Determination of transimpedance feedback resistor Rf

Bandwidth is the most talked about in circuit analysis, but the calculation of zero and pole cannot be separated from the resistor Rf , so the resistor must be determined first. How to choose the value? Everyone wants to get the largest possible conversion rate so that current-voltage conversion can be achieved in one step. However, when Rf is so large that the voltage drop IbRf generated by the op amp base current is close to or even reaches the top power rail, the circuit will be unstable or shut down. Therefore, the feedback resistor value is subject to the base current, and Rf is taken as <1/Ib ; at the same time, the op amp input resistance Ri parameter is taken into account, and the smaller of the two is taken.

captz

captz posted on 2022-4-25 09:57 *), Determination of transimpedance feedback resistor Rf In circuit analysis, bandwidth is the most talked about, but the calculation of zero and pole cannot be separated from the resistor Rf ...

*) , bandwidth

The transimpedance op amp circuit is a voltage follower, a low-pass filter circuit. The amplitude-frequency curve has a raised resonant frequency point at the upper turning frequency of the bandwidth. If the higher harmonics in the signal spectrum are at the same frequency, resonance will occur. If the bandwidth of the circuit is greater than the highest harmonic fmax of the spectrum , it will not occur. The near-ideal rectangular wave spectrum fmax is 217 times the fundamental frequency fo , while this simulation is 101 times, so if GBW>100fi is selected , there will be no resonance, and there is no need to calculate the bandwidth, which is much simpler.

Assuming the equivalent input capacitance Ci=30pF , for the same circuit, the highest harmonic frequency of the 50Hz rectangular wave is far less than the resonance point of the amplitude-frequency curve, and the output signal is not distorted. However, when transmitting a 1KHz signal, the resonance is as shown in the red line.

captz

*) , offset voltage is the key

It is very difficult to automatically adjust the offset voltage by tracking the change of the op amp parameters , but the offset voltage can be adjusted to a very small value, so that the offset voltage caused by the parameter change due to temperature drift error and other reasons is within the accuracy requirement range, so that a large margin is left for the op amp parameter values. According to the data searched on the Internet, if the temperature changes by 100°C , the margins of various parameters, Ib is taken as 100 times, and Vos , Ao , Ri , SR and GBW are taken as 10 times.

I have posted an op amp combination follower before, which can adjust the offset voltage to a very small level. The following is an op amp Lm324 composite follower, with an offset voltage residual value Uo as small as nV level; if the parameters change due to temperature drift and other reasons as shown in the red box, the residual value Uo is only 2uV .

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*) , using a composite follower transimpedance circuit

Add two resistors to the above circuit, and calculate according to the parameters in the red box. Rf realizes current-voltage conversion, and the conversion rate is R=Rf=0.3e+6 . In response to the + -end base current voltage drop IbRf , set the resistor Rf'=Rf to offset the IbRf' voltage drop. Input Ii=10nA , Vo=IiR=(1e-8)(0.3e+6)=3mV , set Ra fine-tuning, and the center axis deviation is uV level.

The input is changed to the connection point of Rf' and Ra , and it becomes a normal inverting input follower. The conversion rate R = -Rf' , and Rf becomes the zero adjustment resistor of the output offset voltage ( as shown in the black line ) .

captz

captz published on 2022-4-26 20:10
*), using the transimpedance circuit of the composite follower,

add two resistors to the above circuit, and calculate according to the parameters in the red box, Rf realizes current-voltage conversion, and the conversion...

*) This post is almost finished. Now there is a simulation question: how to convert a 3Hz/ 10pA rectangular wave current signal into a mV voltage output? The conditions are two op amps Lm324 , the parameters are as shown in the red box above, and only resistors are used to build the circuit.

freebsder

Learned in detail.

Demmo

Very good, I collected and learned