antenna accident

[micael]

Hi,
I m trying to amplify the voltage from a 433MHz flexi whip antenna which is connected on a strip board.
I have connected 5v supply to the antenna as shown in the schematic (..accident) ending with a circuit(on a stripboard) that works at 200MHz, 300MHz and sometimes 420MHz but not at 433MHz!!!!!
Any ideas whats going on with this circuit?

 

[dknguyen]

Stripboards can't work with frequencies anywhere near that high. Too much parasitics. The 741 can't work at that frequency either.

 

[audioguru]

An old, old, old 741 opamp has full output swing up to only 9kHz. Its gain drops to 1 at 1Mhz and keeps dropping at higher frequencies.

Your circuit might be using the opamp's input transistor as an AM detector diode.

 

[RadioRon]

It is hopeless to expect your circuit to work as shown because as mentioned above, the 741 is not capable of amplifying such high frequency signals.

You should be using a part like this one:

http://ca.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=11773&Row=708587&Site=CA

**broken link removed**

It is a surface mount part and the capacitors for bypassing the dc input and coupling RF in and out must also be surface mount parts mounted close to the IC. You can scratch out a board with a knife and small 1 inch by 1 inch scrap of copper board, but use double sided and ground the top to the bottom all around with copper tape.

 

[Analog]

433 MHz - a difficult proposition. Buy a booster amp.

 

[micael]

An old, old, old 741 opamp has full output swing up to only 9kHz. Its gain drops to 1 at 1Mhz and keeps dropping at higher frequencies.

Your circuit might be using the opamp's input transistor as an AM detector diode.

This stands only for open loop or it applies also for inverter amplifier using 741 op amp were u can specify the gain ?

 

[audioguru]

The old, old 741 opamp was designed for amplifying DC. So they made its DC open loop gain about 200,000. To prevent it from oscillating with negative feedback added, they added a capacitor that cuts its gain above 2Hz at the rate of a loss of half per doubling of frequency until it doesn't have any gain (a gain of 1) at 1MHz. The output keeps dropping above 1MHz.

The capacitor also causes its max output to have slew-rate-limiting above 9kHz. The output at higher frequencies is converted to ramps like triangle waves and higher frequencies are reduced.

It doesn't matter if it is an inverting or a non-inverting amplifier. It is lousy at frequencies above only 9kHz.

 

[micael]

Hi audioguru
thanks for replying
can u give me the source of the attached pictures please?

 

[audioguru]

can u give me the source of the attached pictures please?

National Semi makes the LM741 so I got their datasheet. Most other semi companies make a 741 and their datasheets are similar. I got it from www.google.com or from www.datasheetarchive.com .

 

[micael]

National Semi makes the LM741 so I got their datasheet. Most other semi companies make a 741 and their datasheets are similar. I got it from www.google.com or from www.datasheetarchive.com .

Can you be more precise please? i have tried everything on : **broken link removed**
but i couldn't find those two graphs

 

[audioguru]

I guess the 741 is so old that companies don't publish its full spec's anymore.
Try the datasheet for the uA741. I have it from TI and its frequency response graph is with dB's instead of thousands of times for its gain.

 

[micael]

What if LM741 is used as acomparator? (https://www.technologystudent.com/elec1/opamp3.htm)
it will still be the same with frequencies over 1MHz? i mean the gain will keep falling down?

 

[Nigel Goodwin]

What if LM741 is used as acomparator? (https://www.technologystudent.com/elec1/opamp3.htm)
it will still be the same with frequencies over 1MHz? i mean the gain will keep falling down?

The 741 is still a crappy old antique - why would you want to be using just a device in the 21st century? - you can buy much higher spec devices for less money!.

 

[Dr.EM]

In comparator mode your trying to use the full open loop gain (200,000 or so). It will perform terribly at 1MHz, you might end up with a low amplitude triangle wave. Some op-amps will work at 1MHz though, just look for high speed ones.

 

[micael]

comparator

I ve just made this circuit that works at 400MHz....?

 

[Dr.EM]

It doesn't, don't trust the sims . 400MHz is far too high for any op-amp, you need RF amplifiers or the best option, buying a ready made unit. Making anything for these frequencies on stripboard will not be sucessful, not at least without extreme care.

 

[Nigel Goodwin]

I ve just made this circuit that works at 400MHz....?

Whatever you're ingesting?, don't let the police catch you!.

 

[micael]

741

In comparator mode your trying to use the full open loop gain (200,000 or so). It will perform terribly at 1MHz, you might end up with a low amplitude triangle wave. Some op-amps will work at 1MHz though, just look for high speed ones.

i get those graphs when i test the circuit using 433MHz sinewave signal

 

[dknguyen]

What's the scale on those graphs? They are a bit meaningless without scales so the input can be compared to the output.

 

[micael]

What's the scale on those graphs? They are a bit meaningless without scales so the input can be compared to the output.

For the first, the antenna input is 23mV Pk-Pk
For the second, that is amplifier output is 3.23V Pk-Pk