The circuit shown in Figure 3 has the following transfer function. The transfer function of the current source has A/V = Mho units, so this is a transconductance amplifier. This circuit shows a simple RC network with its cutoff frequency at 1/(2 π RC). We could introduce a pole as in the following figure. To simulate this we need to introduce this pole in our SPICE model. Starting with the cut-off frequency, the open loop gain versus frequency plot has a drop of 20dB for every decade of frequency. This frequency is not identified in the datasheet, but can be easily calculated from the open-loop minimum gain of 500000 and the gain bandwidth product of 12 MHz. In that article I showed that ADA4004 has a cutoff frequency at 24 Hz. See this article for more details: :An Op Amp Gain Bandwidth Product. Op amps have a dominant pole, inserted by manufacturers on purpose, so that the op amp is stable at any gain down to zero dB. The Gain Bandwidth Product, describes the op amp behavior with frequency.
Let’s continue building this model to simulate the Gain Bandwidth Product. As an example I chose Analog Devices’ ADA4004. Part 1 of this article ( ) shows how to create a behavioral model of an operational amplifier based on the following parameters found in the datasheet: Input and output resistance, input capacitance, DC gain, and offset voltage.
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