I want to use AD811AN as high frequency amplifier. According to its datasheet , Its Absolute maximum V+ and V- are +18 and -18 and differential input voltage is +/-6 V. Most of the graphs in the datasheet are tested in +/-15 v supply voltage.
When I use it with +/-12 Volt supply and input signal= 0-50MHz , 200mV pk-pk (which are all well below the maximum ratings), it works fine but gradually it gets very hot (in around 5-10 minutes) until stops working.
Here are thing I tried:
1- Removed all components until it was just connected to V+ and V- . all input, output and ground connections removed. but it gets hot.
2-Replaced it with 3 other parts(with the same part number ) but they got hot also.
I should mention that after the device gets cool, it works fine again for next 5-10 minutes.
I am sure there must be something that I am missing with this device. Please let me remind that the device gets hot even when just pin7=V+ and pin4=V- are connected to 12V and all other pins are not connected to anything.
Best Answer
This amplifier is designed to operated in a specific fashion with specific feedback resistor values. Note that the In+ and In- terminals have very different characteristics. Operating the device un-connected w/o feedback circuits it is unsurprising that it has problems. To guess what is happening would be just that, guessing. Connect it up with the recommended values, Do not leave the input floating (it will pick up stray fields and amplify those and chew up more current doing so) and then if it's over-heating come back and ask the same question.
These devices can run hot. the quiescent current (taking worst case @ +/- 15 V) is 18 mA. This is dropped across 2*12 = 24 V => 0.432 W of dissipation. in a DIP package it it 90 C/W from ambient 39 C + 25 C (room temp) = 64 C even before it is driving anything.