# Choosing a suitable Opamp to output a voltage reference

Part 1: the circuit and how it's connected

I'm trying to take from the 2.5V coming from a voltage reference IC AD780 to go through a unity gain buffer then go through a 2x Gain opamp stage to finally output 5V with a worst case scenario of current sinking 25 mA . Bear in mind that this application is part of a system that will be placed behind the back seat in a car so i believe ambient temperature may be around ( 40 - 50 °C ) .

Part 2: my conclusions so far

Here's what i finally came up with after searching and filtering through a lot of rail to rail o/p op amps :

Part 3: my questions

1. Which one is most suitable for the application. If none please suggest and state why (so others may benefit).
2. I figured I'm just using a DC signal (I believe) and not AC one so I'm a bit confused to which parameters do I need to worry about in the datasheet.

Part 4: schematic ( Simulation using Proteus Isis ) I did a simulation for the LMC6482 and it worked fine with 25 mA load but after all this is a simulation and other real factors (temperature, other parameters that I am unaware of ... ) .

Rather than searching for an op-amp that can source 25mA on a rail to rail output (although your supply is apparently 3V above the desired output of 5V), use an outboard boost transistor.

The circuit design is straightforward:

1. Determine the maximum current from the op-amp across temperature that will not generate excessive heat in the device (most general purpose devices have a rather high $\theta_{jc}\ and \ \theta_{ja}$).

2. Calculate the required minimum transistor $\beta\ as\ \frac {I_{out}} {I_{opamp}}$ across temperature and choose a suitable device. The BC846 I have used is well known for its high current gain characteristics.

3. The power in the boost transistor determines the package, primarily: so for your circuit we have 75mW dissipation in the transistor. For one of the packages for the BC846 I have used, that equates to a temperature rise of 37.5C; if your environment is at $\le$ 50C, that yields a junction temperature of 87.5C, well within the capability of the device, although it can shorten the life of the device. If that is a concern find a device in a chunkier package.

The series resistor I have used yields acceptable performance, but it could be reduced a bit at the expense of more power in the op-amp output stage.

This approach permits the use of general purpose op-amps rather than specialised parts, which should reduce cost as a bonus.

The circuit you are attempting to design is undemanding in most ways for the opamps you've selected, except in the output current capability. You say you want to sink 25mA, but the opamps may not be capable of supplying that much output current, depending on the supply voltage, output voltage, and temp. The datasheets give example graphs for Vs = 3, 5, and 15 V, and minimum specs for V+ = 5 and 15 V, but your supply is 8V, so you need to interpolate the given specs. Also bear in mind the back seat of a car could also be -30˚C also (if you live in a climate like mine). The minimum spec over extreme temps for V+=15 volts is 24 mA sinking ability, so this op amp most likely will work for you, but it's not guaranteed to work in a "four corners" robust design sense. If this is a one-time design that you will build one of, it's OK. If you will produce a million of them in a production line, you will have some failures.