Ok so my question is what resistor values should I choose for this amplifier. Vin is 0.5Vpp, 50kHz and the desired Vout is 25Vpp, so the overall voltage gain is 50. I know that the voltage gain for an inverting amplifier is -Rf/Rin. What's the difference between choosing resistor values like 50ohms and 1ohm or like let's say 50kohm and 1kohm. Also, what should my +-Vcc supply voltage to the amplifier be? The 2nd picture below is the op-amp's(LF351) datasheet Wondering if that could affect anything. Any help is much appreciated, thanks.
This should help you decide on the power rails: -
And this graph just about permits a gain of 50 with a load of 2 kohm: -
Regards resistor values, I'd be thinking of 22 kohm for the feedback resistor and about 440 ohms for the input resistor. You might be able to go a little higher and still get the 50 kHz bandwidth you need. Maybe as high as 220 kohm for the feedback resistor.
What's the difference between choosing resistor values like 50ohms and 1ohm or like let's say 50kohm and 1kohm.
The op-amp is inverting hence the inverting input is at 0 volts hence the output load IS the feedback resistor and you can't have this too low or you won't get the output voltage amplitude. On the other hand, you can't go too big because the parasitic capacitances of the op-amp will start to reduce gain too much at higher frequencies.
Regards numbers in the data sheet (rather than graphs), this is a summary on page 4: -
The supply is listed as +/- 15 volts and the output swing is typically +/- 13.5 volts. This means that you can't push the output to typically within 1.5 volts of the positive or negative power rail but, this could be as bad as 3 volts if you take the minimum figure. The 1.5 volts and 3 volts numbers are saturation voltages and the load is 10 kohm.
Let us start with an example, to get comfortable.
I by default use 10,000 ohm resistor as basic value; thus 100Kohm would be the Rfeedback value for inverting gain_of_10. With 10 volt output, only 0.1mA (100uA) output current is needed. And the total resistive_contribution noise, assuming 1MHz UGBW and thus 100,000Hz opamp+resistors bandwidth because of gain of 10X, will be (approximately)
- 4 nanoVolt/rtHz * sqrt(10Kohm/1Kohm) * sqrt(100,000Hz BW) * Av=10
- 120nanoVolt * sqrt(100,000) = 120nV * 316 = 36 microVolts RMS
Thus the opamp is comfortable with an easy load. And the output noise is 36uV rms, spread over the entire bandwidth of 100,000 Hz.
The picking of resistor values is constrained by
Operational Amplifier output current and heating; many opamps have a SHORT CIRCUIT protection feature, usually about 10mA to 20mA; read the datasheet
random NOISE generated inside the resistors you choose; the wider the bandwidth, the more noise, which increases as the square_root of the bandwidth. The noise_power increases linearly with frequency; the noise_voltage increases as the square_root; remember Power = Voltage^2 / Resistance.
the PCB parasitic capacitance and the OpAmp input capacitance will interact with the resistors you pick, to limit the bandwidth and to cause phase shifts that may cause ringing and poor settling and even oscillation.
High resistor values introduce more feedback phase shift, where the C_input_differential and PCB parasitic capacitance will matter.
If frequency response peaking, and time_response ringing, and possibility of oscillation, do not make you nervous, then you can ignore this.
An opamp with higher phase margin may be your friend.
remove any ground plane under or near the summing node PCB foil; that node is the Vin- of the opamp
move all components of summing node very close to Vin-, so minimal PCB area is used
enlarge the solder pads of the Rfeedback, and have them just barely separated (almost shorting), so the electric fields across the Rfeedback are maximum
have a tiny capacitor across Rfeedback (some people use trimmer-caps; you could use a "gimmick" cap made of 2 wires twisted together (insulated from each other)
replace the Rfeedback with a "T" attenuator of much lower Rvalues
pick an opamp with lower input capacitance (some opamp datasheets do not provide this information)