Using an op amp as a buffer

[theguy]

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.

 

[barry]

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.

 

[theguy]

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.

 

[Pjdd]

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.

 

[jiripolivka]

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.

 

[theguy]

@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?

 

[goldsmith]

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

 

[jiripolivka]

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

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.