Amplify small pulse signal

[MerlinKnight]

Hello,

Could someone tell me how to amplify a neg sawtooth pulse.
I want to pick up a pulse with an ant wire that varies from -800mv to -1.4v. The width between pulses is 16ms. Pulse width is 450us. I want to convert this to varying 0 to +5 volts.
Any help would greatly be appreciated. Started with some transistors but not able to achieve what i want.

Thank You
jim

 

[BrianG]

Jim, a bit more information would help -
1) what is an "ant wire" - antenna(?), which seems a bit unlikely as the highest frequency is only a few kHZ.
2) Is the "-800mV to -1.4V" the signal on this "ant wire", or is it at the output of whatever circuit produces the sawtooth?

 

[logbook]

Since you are a "computer" person, I presume you are going to feed the signal into some sort of logic gate. What you are then looking at is more like a comparator than a linear amplifier. You need to clip the signal to stop the logic gate being overloaded. It is also bad to run through the logic gate threshold too slowly.

It should be very straightforward to amplify this signal by x4 to give the required 5V swing. This would easily be accomplished using a single transistor amplifier. However, if you are not familiar with transistors then you may be better to just use an opamp. Run the opamp from say +5V to -5V, make the feedback resistor say 3K9 and the input resistor say 1K. Wire the + input to 0V. Wire the - input to the output using the feedback resistor (3K9) and feed the input in via the IK resistor.

OK, now that has given an input impedance of 1K, which may be too low. You can increase the resistor by a factor of 100 if you need a higher input resistance, but you may need to put a 10pF capacitor across the feedback resistor to stop the opamp from oscillating.

 

[MerlinKnight]

1) what is an "ant wire" - antenna(?), which seems a bit unlikely as the highest frequency is only a few kHZ.

The ant is just a paper clip at the moment.

2) Is the "-800mV to -1.4V" the signal on this "ant wire", or is it at the output of whatever circuit produces the sawtooth?

The voltage reading is from this ant. I am reading this on my scope.
The signal i am looking for is a analog out of 0 to 5 v. I am going to feed this into a meter/leds to read strength of signal to pinpoint max signal area.

This was suppose to be an easy circuit. But i am more familliar with logic circuits , work with them all the time, than analog. Having to break out the school books.

Signal looks something like this

-----------| /----------
| /
| /
|_/

 

[felixc]

Your scope probe is high impedance. What I suggest you to do first, is to look at using an op-amp with FET inputs to buffer that signal, and then convert that signal to what you need. Otherwise whatever you connect to your ant will produce a different waveform.

You connected the scope probe ground to the ground of your circuit to check?

 

[Comcokid]

Sounds like you are trying to amplify and then detect a signal level. Why not look at standard AGC diode detector circuits, or something along the lines of an Analog Devices AD8361, or one of their other detector chips.

The AD chips have enough gain that you would need a smaller antenna, or attenuate the signal before the chip.

 

[MerlinKnight]

Thanks for the help.

I will try the ad8361 chip. I did have it grounded to the circuit. I will also try the op-amp suggestion and any others i can get.

Jim