Need to repair a legacy system that has been discontinued and am considering replacing the PNP based circuit with the diode based circuit.

The input signal (max. freq = 20Khz) is applied at IN and is active low. It's TTL level.

The output is tapped from OUT and 3.3V is applied to this point through a weak pullup.

PNP based circuit diode based circuit

  1. Given this information, are both the circuits equivalent as far as IN and OUT digital signals are concerned?
  2. Is the PNP based circuit effectively a 3.3V level convertor?
  3. Is the diode based circuit an equally effective 3.3V level convertor?
  4. Why would a company prefer the PNP based circuit over the diode based circuit?
  • \$\begingroup\$ The diode based circuit has an error: there is no connection fron Cathode (Vin) to ground. \$\endgroup\$ – sekharan Apr 13 '13 at 4:21
  • 1
    \$\begingroup\$ The diode based circuit has an error: V1 is shorted to ground. \$\endgroup\$ – jippie Apr 13 '13 at 8:22
  • \$\begingroup\$ @jippie: Please disregard that short. It does not exist and was drawn in error. \$\endgroup\$ – sekharan Apr 13 '13 at 8:24

Given the latest information, see answer below.
As information is being supplied in occasional small dollops the answer may well change again with time.

Once all data has been provided the correct answer will be able to be given if anyone is still answering.

Hint: Tell all you know up front, place known voltage on diagram. If there is a lnown IC driving it show the IC etc.
If you think the IC is irrelevant to the question then you must know more abnout the answer than is apparent.

Given information as at about April 14th 2013, transistor and diode circuits are similar in effect.
Transistor will provide more current pull down but this should be reasonably irrelevant. Using a Schottky diode will give lower Vlow_out.

The circuit IS effectively a level shifter. Vout max = 3V3. Vout min is Vinlow + Vbe or Vdiode.

  • \$\begingroup\$ Thanks for catching the error early. The diode based circuit has an error: there is no connection fron Cathode (Vin) to ground. There is not load. OUT goes to a microprocessor pin(and the firmware treats that pin as an input) \$\endgroup\$ – sekharan Apr 13 '13 at 4:23
  • \$\begingroup\$ The input signal (IN) has 5V DC amplitude and is actually the output of TSOP1738. The output signal (OUT) goes to a 3V3 microprocessor. Hence my question as to if these are actually level convertors. The first circuit is in production circuits in Toshiba equipment already out in the wild. \$\endgroup\$ – sekharan Apr 14 '13 at 2:34
  • \$\begingroup\$ This question has become messy. I have asked it again with proper title and text body. \$\endgroup\$ – sekharan Apr 14 '13 at 3:16

No, the two circuits are not the same, even after ignoring the dead short of the input in the second circuit.

Both circuits will produce about the same output for a valid input voltage. The difference is the load presented to whatever is driving the input. In the top circuit, the transistor is used in what is called the emitter follower configuration. Most of the output sink current goes to ground thru the transistor. In the bottom circuit, all the output sink current is sunk by whatever drives the input.

The bottom circuit is like cranking a shaft manually. The top circuit is like controlling a throttle so that a engine turns the same shaft at the same speed for you.


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