anyone suggest circuitry for CMOS to TTL logic conversion

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kushal.khanal

Joined: 20 Aug 2007
Posts: 14

23 Aug 2007 17:14 anyone suggest circuitry for CMOS to TTL logic conversion

Hi again guys n gals!!!

As shown in attachment below,

what the problem is input to opamp is 6-8v DC which is first case;
now since opamp is used as a comparator, it gives 4-5V output,which is desired;

BUT, even when input to the comparator is 200-300 mV still output is 4-5V.

Now, the more interesting fact is that if I supply the comparator 6-8v DC from a separate voltage source, comparator gives 4-5V output which again is desired.

Now,when input to the comparator is 200-300 mV from a separate voltage source, output is of the order of 200-400 mV,which is absolutely correct.

But, obviously I can't take 6-8V input from a separate voltage source(I have to use the 6-8V DC generated within my circuit.

Is it the current that is playing underground role??????

But since I have used 7809 voltage regulator as supply and there is only one IC before this stage;So I don't think current is the cause.

So what should I do??????????
Any similar experiences??????

Regards,
Kushal Khanal

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AA1LL

Joined: 23 Aug 2007
Posts: 2

23 Aug 2007 22:28 Re: anyone suggest circuitry for CMOS to TTL logic conversio

The input voltage divider should be at vcc/2 for cmos input compatibility. Put two resistors of equal value as a voltage divider here, the junction going to pin 2 of the op amp (1IN-). Use 10k, 1meg is too high.

The output should be able to sink 1mA or so at logic=low and look like an open circuit (or several k) when high for the output to look like a TTL gate output. Put a diode cathode (the arrowhead) directly to pin 1 of the LM358 and the anode to Vcc through 5k. The TTL output will be at the anode of this diode. Remove the other two resistors at the output.

kushal.khanal

Joined: 20 Aug 2007
Posts: 14

24 Aug 2007 4:56 Re: anyone suggest circuitry for CMOS to TTL logic conversio

AA1LL wrote:

Hey there AA1LL

May be i couldn't make you understand.
Or you might have forgot
1.Are you clear that the IC LM358 perfectly works as a separate module even with the circuitry as in the attachment;

But in the circuitry where it is to take the input 6-7 v or 200-300mV from circuit
it doesn't works as wished?@?@?!!!

What might be the cause ?

2. And since in LM358 biasing voltage is 9V .Did you regarded this as Vcc ?
Or can I use Vcc as any other voltage.I think I can .

Regards,
Kushal Khanal

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amirs4

Joined: 23 Aug 2007
Posts: 15
Helped: 1

24 Aug 2007 6:43 anyone suggest circuitry for CMOS to TTL logic conversion

i think u must use a voltage divider at the input.,

spectrabei

Joined: 24 Aug 2007
Posts: 30
Helped: 1

24 Aug 2007 11:46 Re: anyone suggest circuitry for CMOS to TTL logic conversio

First of all, you don't need the cap on the inverted input of this circuit.
Second, this is a bipolar device, the current at the base (the noninverted input) has to be above 6 microamps. I would check the circuit that feeds the non inverted input.
Note : you have 8.59 microamps on the inverted input of this circuit. this is ok