gain-bandwidth-productoperational-amplifier
Below is the schmatics and AC transfer function for transimpedance amplifier from this design.
According to the datasheet, OpAmp has the gain bandwidth product of 20MHz.
But, here the gain is 94.58 dB = 54954 V/V and the bandwidth is 1.464 MHz.
So, Gain * BW = 54954 * 1.464 MHz = 80452 MHz, which is a lot than the value specified in datasheet. Can anyone help me understand how such high GBP is possible here?
ConfusedCheese, here is a simple method for simulating the loop gain for an operational amplifier with feedback. This simulation scheme provides DC feedback and a stable operational point (for double and symmetrical voltage supply only!).
The dashed box contains the feedback factor, if any!.
Without this box (direct connection to the inv. input) you simulate the open-loop gain Aol of the opamp only.
The loop gain is simply T(s)=V(node2)/V(node1). Of course, the frequency must be swept over the desired range (ac analysis).
simulate this circuit – Schematic created using CircuitLab
Gain-bandwidth product is a small signal specification. It only applies to signals under which the op-amp circuitry remains in a linear regime. That is on the order of 100mV or less for a conventional differential input stage.
With the relatively high frequency 2V you are applying you are well into a large signal regime in which non-linear effects take precedence. The specification that applies in that regime is the slew rate of the amplifier.
Slew rate limitations are caused by how fast the internal bias currents are able to charge the internal (compensation) capacitances.
Best Answer
The TransImpedanceAmplifier (TIA) has a gain (volts out to current in) equal to the feedback resistor. It is 53.6 kohm and take the log of this and multiply by 20 and you get 94.58 dB.
Gain bandwidth product is all about voltage gain - you don't have any voltage gain in a theoretical TIA circuit unless you are going to perform noise analysis.
If you were to analyse noise you'd realize that without the feedback capacitor (2p7) the amplifier's input's "self noise" is significantly amplified at higher frequencies due to the parasitic capacitance of the photodiode. It basically forms a gain stage from the equivalent input noise in series with the non-inverting input.
The 2p7 seeks to reduce this effect by progressively shunting the 53k6 as frequencies rise.