I am quite new to op-amps but have looked at them in college and have done my own research on them. I am trying to use an op-amp to build a simple band-pass filter which has an in-band voltage gain of 5. The main goal is to have a signal which is received from an ultrasonic transducer enter this filter. The signal will be a sine-wave in the order of a couple hundred millivolts around 1.1MHz frequency. I want the band-pass filter to limit the noise that passes through. I also want the in-band gain to be around 5.
Let me go through what I have tried already. I have been looking at Texas Instrument’s TDC1000 which is an “Ultrasonic Sensing Analog Front End”.
I have been looking in particular at the Low Noise Amplifier in the receiver path. I am trying to replicate the circuit seen in Figure 16 on page 14:
According to the datasheet, the in-band gain is set by the capacitor ratio:
Gain_in-band = CIN/CF = 300pF/30pF = 10
The high-pass corner is set by the feedback resistor and capacitor:
FCH = 1/(2πRFCF) = 1/2π(9000)(30x10^-12) = 589.5kHz
The low-pass corner is set by the Gain Bandwidth Product and the gain:
FCL = GBP/Gain = 50MHz/10 = 5MHz
This all seemed to make sense, however, other websites showed different circuit configurations for band-pass filters. Some circuits included another resistor before CIN and there was no mention of the "Gain-Bandwidth Product".
I decided to build my own version of the Low Noise Amplifier circuit that was presented in the TDC1000. Picking out an op-amp was tricky enough but after doing some research, the LT1128 seemed like a good choice. The datasheet for this op-amp can be accessed below:
It is a low noise precision high speed op-amp with a Gain-Bandwidth product of 20MHz. I built the following circuit on a breadboard:
Following the previous equations:
Gain_in-band = CIN/CF = 100pF/20pF = 5
FCH = 1/(2πRFCF) = 1/2π(10000)(20x10^-12) = 795.8kHz
FCL = GBP/Gain = 20MHz/5 = 4MHz
To test out the circuit, I applied a 500mV peak-to-peak sine-wave from my signal generator and varied the frequency. The circuit was not operating as I had expected. Below are a few screenshots from my oscilloscope. The yellow channel is the input signal, and the blue channel is the output of the LT1128.
I am not too sure what is going on, changing the amplitude of the input signal did not change the amplitude of the output. However, varying the frequency of the input signal had an affect on the phase and amplitude of the output signal. What I had expected to happen was that within the frequencies from 795.8kHz to 4MHz the output signal would be inverted and 5 times greater than the input.
I decided to take a step back and just build an inverting amplifier with a gain of 4.7. Below is a schematic of the simple circuit that I built on a breadboard:
I applied a 1V peak-to-peak sine-wave and varied the frequency. At 300kHz the output signal is as expected, it is inverted and has a peak-to-peak voltage of 4.7V. Below is a screenshot from my oscilloscope, yellow channel is the input signal and the blue channel is the output:
As I increase the frequency, the phase and amplitude of the output signal changes and the amplitude of the input signal no longer has an effect on the output.
I am confused as to why this happens, I thought that the op-amp should continue to amplify the input signal at much higher frequencies.
I have done lots of research on the bandwidths of op-amps, but it seems that I am missing something.
- Is the op-amp itself that I picked not right for this application, is it one of the parameters of the LT1128 that I may have overlooked?
- In regards to the band-pass filter circuit, can anyone spot any mistakes with the schematic or can explain what is happening?
- In the inverting amplifier configuration, why does the output signal start to change above an input signal of 300kHz?
If it’s the fact that I need to do more reading, does anyone have any recommendations?