Using an op amp as a buffer

I have a 1Hz square wave provided by a GPS receiver (1pps). I need to input this into a system that has a 50ohm load, and the GPS receiver cannot provide the required current so I need a buffer. I have a LM386N Audio Amplifier handy and was wondering if it would be capable of acting as a buffer. The square wave is a 0-5V signal.

The datasheet for the LM386 does not show any output sink ratings other than a 250mW output power with a load of 8ohms. That comes to 177mA which is more than enough to drive the 50ohm load @5V.

I tried radio shack but they had no op-amps that could sink 100mA. I was hoping to avoid ordering the part and waiting for the part to be shipped.

Any recommendations are welcome.

I'm not sure you'll get a square wave out of the LM386. Also, you're asking for 500mW-can the LM386 handle that (5V/50ohm)? Or you could use a 'regular' op amp, and add a bipolar transistor stage on the output.

good point, it's been a while since I've messed with circuits, even the small and simple ones such as this. I think the max out power is 400mW so that wont do. I was also concerned with the output being able to produce a clean square. The problem was finding a regular op-amp that could source 100mA. something like an LM741 can do ~25mA. I have some milspec parts, but I am having trouble finding datasheets for them. UA723ML and OP12500.

Some thoughts on the idea of using an LM386 -
It's capable of amplifying the audio spectrum with reasonable fidelity, so I imagine that it could produce fair rise and fall times at 1Hz. IOW a reasonably square wave.

The LM386 can drive a 4-ohm load with audio signals at Vcc of 5-12V. So I'd think a 50-ohm load would be a breeze. What I'd consider is the saturation voltages. With Vcc of 5V, the datasheet curves indicate a p-p voltage swing of about 3.5V with a 16-ohm load, and 3.8V with an open load, say about 3.6V with 50 ohms. Assuming symmetrical saturation characteristics, this means a Vce(sat) of +/-0.7V. Will this be a problem with the intended application?

Vce(sat) of 0.7V also means that the sink/source load current with a 50-ohm load is 86mA. This means a max dissipation of 60mW. The DIP version is rated for a dissipation of 1.25W at 25 deg C.

Possibly the simplest solution for this problem would be to use a "power optocoupler" like PVG612. Its input is a LED, the external resistor establishes the load current from your GPS receiver. I used to send only 3-5 mA in the LED; the output can carry a 2 A load at 60 V max., so you can connect even more than one 50-Ohm loads in parallel.
There are more types of "power optocouplers" on the market. No power supply required, you can use the +5 or +12 supply for the GPS signal source to set the current to your 50-Ohm load.

@Pjdd: That would likely be fine as I'm just looking to feed a TTL 5v input.

@jiripolivka: The recommended connections confuse me a little. How would you use the PVG612 to drive a 5v TTL 50 Ohm load with a 1Hz square wave?

Dear theguy
Hi
The answer of your question is not complicated . as i understood , you wanna , make 5v 1HZ on 50ohms load? if yes , as barry said , you can use a complementary pair at the out of your opamp . but if you don't want to do thing that he said , i can introduce you a expensive way , why you didn't use a simple LM12 ( 800W op amp )?
Best Wishes
Goldsmith

goldsmith said:

Dear theguy
Hi
The answer of your question is not complicated . as i understood , you wanna , make 5v 1HZ on 50ohms load? if yes , as barry said , you can use a complementary pair at the out of your opamp . but if you don't want to do thing that he said , i can introduce you a expensive way , why you didn't use a simple LM12 ( 800W op amp )?
Best Wishes
Goldsmith

Click to expand...

The PVG612, "power optocoupler" has an isolated input and MOSFET output, and operates like any optoisolator. From your source (TTL pulses, 1 Hz) connect the input LED through a ~1 kOhm resistor, and ground. The output MOSFET will open whenever the LED is lit. As the MOSFET is isolated, use the +5 or +12 V DC you power the GPS receiver from, and connect it via a suitable resistor through the optoswitch to your 50-Ohm load. The dropping resistor can be such that you get e.g. +5 V across the 50-Ohm load, so you need ~100 mA from your power supply.
The PVG 612 I used can hold 2A at 60 V max.