# Simple OpAmp selectable gain amplifier: Select gain 1x or 10x via GPIO

This circuit is a simple variant of a selectable gain amplifier.

If CTRL is pulled low (i.e. to GND), effectively the circuit will look like this:

Due to negligible input bias current for JFET opamps like the TL084 (i.e. most modern opamps below 100 MHz GBW), this circuit is basically just a Gain 1 buffer.

If CTRL is pulled high(i.e. pulled to 5V for example), the circuit effectively looks like this:

which has a gain of ($\frac{10 k\Omega}{10 k\Omega + 90 k\Omega} = \frac{10 k\Omega}{100 k\Omega} = 10$. Adjust these

One aspect to consider here is the RDSon of the MOSFET Q1. This is added to R1 and introduces a gain error to the system. The BSS123, for example, has a relatively high RDSon of up to 10 Ohms at VGS = 4.5V. For R1 = 10kOhm, the effective worst-case gain is:

$\frac{10 k\Omega + 10 \Omega}{10 k\Omega + 10 \Omega + 90 k\Omega} = \frac{10 k\Omega}{100 k\Omega} = 10.001$  or 0.01%. For most applications this does not matter since other aspects like the resistnance tolerance or resistance coefficient of temperature will affect the system error orders of magnitude This gain error will vary with temperature since the RDSon is temperature-sensitive and often the variation will be a larger issue than the gain error itself.

One way to mitigate is to choose large values for R1 and R2. Choosing R1=100kOhm and R2=900kOhm in this example will reduce both the gain error and the gain error temperature coefficient from the RDSon of Q1 by an order of magnitude. I recommend not to choose resistors larger than 1 MOhm without taking further consideration of opamp bias current, bias current temperatur drift etc, since those start to get relevant even for JFET input opamps when exceeding a couple of Megaohms.

Note that the RDSon of the MOSFET will be worse for VGS=3.3V will be worse and some MOSFETs will not switch on properly at this voltage. Choose a MOSFET that fits both your logic voltage and the RDSon i.e. gain error requirements. I often start with the BSS138 since I have many of those in stock, which tends to have less than 1 Ohm resistance in most practical conditions.

Another consideration for this circuit is that the MOSFET body diode will conduct if the voltage at the inverting input of the opamp will go beyond the Vf of the MOSFET body diode (typically you should use 0.7V here even if the specified body diode voltage is slightly higher, in order to have sufficient headroom). In effect, this typically means that you should not use this circuit without modification when using bipolar supplies and signals that go more negative than GND.