741 Op-AMP help

[Frosty_47]

Hello,

A lot of digital IC's output 3.4V for high instead of 5V. According to my calculations, the circuit below should output 5V. Will the 741 OP-AMP below be able to conver the 3.4V TTL to 5V ?

**broken link removed**

Thanks

P.S. Vcc+ = 12V, VCC- = GND

 

[Frosty_47]

Oh, and are there any "unusually high voltage" Op-AMPS ? Basically I want to transform a 5Vpp Sine Wave to 24Vrms (roughly 68Vpp).

Any part #'s would be greatly appreciated...

Thanks

P.S. I don't want to use a transformer...

 

[synthius]

that circuit will output 5V when a 3.4V input signal is used.

Can I ask what your application is for the 24Vrms output voltage is?

If you are trying to design a audio amplifier then the LM3886 and LM4780 are viable choices.

 

[bananasiong]

I'm not sure whether single supply works for a 741 or not. Why not use modern op amp that capable of being supplied with single supply as low as 2.7 V, with rail to rail output. So you can get 5 V at the output without feedback resistor with the supply voltage of 5 V.

 

[synthius]

he's right, you are better off using a schmitt trigger or even a comparator op amp. 741 would not work because its voltage headroom.

 

[audioguru]

The lousy old 741 opamp is 40 years old and is too slow to amplify a digital signal.
It needs a negative supply. I think without a negative supply then its low output voltage is about +1.2V which is not low enough for TTL but is OK for Cmos.

An OPA445AP opamp from Texas Instruments has a max operating supply voltage of 80V to 90V. They are sold out again. They make about 1000 at a time then they sell out in a couple of weeks.

 

[Nigel Goodwin]

A lot of digital IC's output 3.4V for high instead of 5V. According to my calculations, the circuit below should output 5V. Will the 741 OP-AMP below be able to conver the 3.4V TTL to 5V ?

Why would you want to do this?.

 

[ericgibbs]

I would suggest that you look thru this link, reference switching levels of logic families, before you consider using any level shift/boosting..

http://www.interfacebus.com/Logic_Design_Top.html

 

[Frosty_47]

Thank you folks for your expertise on this matter.

If I redesign the circuit to output 5.7V and put a diode in series at the output, I should than have approximately 0.5V for low and 5V for high ?
I tried searching for other Op-Amps however, it turns out that there are more Op-Amps than empty beer bottles at my place on a Saturday night...

Oh, and a BIG thanks to Audioguru for OPA445.

 

[Frosty_47]

he's right, you are better off using a schmitt trigger or even a comparator op amp. 741 would not work because its voltage headroom.

74LS14 Schmitt Inverter outputs 3.4V for high....

 

[ericgibbs]

hi,

Perhaps If I redesign the circuit to output 5.7V and put a diode in series at the output, I should than have approximately 0.5V for low and 5V for high ?

Are you sure this idea is going to give you what you are expecting.???

 

[Nigel Goodwin]

74LS14 Schmitt Inverter outputs 3.4V for high....

You still haven't answered my question?.

"Why do you want to do this?"

 

[Frosty_47]

You still haven't answered my question?.

"Why do you want to do this?"

Because I am crazy thats why

Basically I want a 5V TTL to go into a MAX7400 that will output a nice 5Vpp Sine wave that I can further boost to 24Vrms with OPA445 (assuming I get one). The minimum High input Voltage for MAX7400 is VDD-0.5V which means it has to be at least 4.5V if VDD = 5V. Thus the typical output of 3.4V will not due for MAX7400 (please correct me If I am wrong)...

 

[Frosty_47]

hi,

Are you sure this idea is going to give you what you are expecting.???

Eh, if I was sure I wouldn't be asking you guys questions

 

[ericgibbs]

Eh, if I was sure I wouldn't be asking you guys questions

Hi Frosty,
Do you have a electronics/logic simulation program in your PC.?

 

[Frosty_47]

Hi Frosty,
Do you have a electronics/logic simulation program in your PC.?

Yes I got Multisim, Quartus II, and Xilinx...

I already tested the circuit in Multisim however I wanted to hear expert's opinion on this topic as I generally don't trust simulators...

 

[ericgibbs]

Yes I got Multisim, Quartus II, and Xilinx...

I already tested the circuit in Multisim however I wanted to hear expert's opinion on this topic as I generally don't trust simulators...

Hi,
I agree, sims are just a guide, but they can highlight errors.

Looking at the MAX7400 datasheet says, Vin can be Vdd/2.

So a 3V input will drive it OK, if its powered from 5V..

 

[audioguru]

The MAX7400 is a Cmos filter IC. It needs to be clocked with a signal that is 100 times higher than its cutoff frequency by a Cmos square-wave or by its own internal oscillator. The clock signal must be with a low of 0.5V or less and a high of 4.5V or more.

The input signal might need an RC lowpass filter to remove high frequency harmonics that would cause aliasing with the oscillator. Your input square-wave might have harmonics that must be removed ahead of the filter IC. The input level to the filter IC can be any level up to about 4.8V p-p but signals lower than 1V p-p have higher distortion and noise at the output. The gain below the cuoff frequency is 1.0.

Why not use Cmos logic ICs to make a 5V p-p square-wave instead of old TTL that makes a square-wave around 3.4V p-p??

 

[Nigel Goodwin]

Because I am crazy thats why

Basically I want a 5V TTL to go into a MAX7400 that will output a nice 5Vpp Sine wave that I can further boost to 24Vrms with OPA445 (assuming I get one). The minimum High input Voltage for MAX7400 is VDD-0.5V which means it has to be at least 4.5V if VDD = 5V. Thus the typical output of 3.4V will not due for MAX7400 (please correct me If I am wrong)...

I'm not familiar with that Maxim part, but if it's supposed to be a logic input it will probably work?. Failing that a simple open collector buffer and a pullup resistor to 5V is all that's required.

I've just had a read of the datasheet, I would suggest you simply try it, I suspect it may be perfectly fine.

 

[Frosty_47]

The input signal might need an RC lowpass filter to remove high frequency harmonics that would cause aliasing with the oscillator. Your input square-wave might have harmonics that must be removed ahead of the filter IC

The input frequency of the square wave is variable (600Hz - 6Khz). What design of low pass filter can I use (if needed at all) for this aplication ?

Thanks