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[SOLVED] Differential signals generation

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Rahul Soni

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Can anyone please tell me if there is any way we can generate the differential signals using BJT pairs?
I know the we can convert single ended signals into differential signals using Op-Amps and some other circuitry. But is there any way to generate Differential signals using BJTs ( By means of some phase difference.)?

Thanks in advance.. :)
 

KlausST

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Hi,

there are many ways..

just use an emitter folower.

But in detail it depends on your specifications: precision, frequency range, DC stability.....

Klaus
 

Debdut

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At the base of one BJT of the pair give dc bias to operate it in active region. At the base of the other BJT give the same dc bias using a resistor. Apply the signal, whose differential you want to create, to the base of the 2nd BJT using a capacitor. Use resistive loads as the loads of the differential pair. Now at the collectors of the 2 BJTs you will see 2 differential signals, one in phase with the input and other out of phase with the input.
 

Audioguru

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A differential signal has two outputs, each output is 180 degrees out-of-phase (inverted) to the other output. To make a differential signal you make one output and simply invert it for the other output.
At a high frequency the inverter will have a delay then the outputs will not be 180 degrees out-of-phase, then you will also need to delay the original phase maybe with a buffer BJT.
 

SunnySkyguy

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Differential Output config is pretty standard.
They use CC on emitters for better performance.
IS there something specific? extended BW? lower Zout?
 

BigBoss

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If you wish produce a differential signal, a cross coupled oscillator is a good way to create differential signals due to its nature.
 

Rahul Soni

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Differential Output config is pretty standard.
They use CC on emitters for better performance.
IS there something specific? extended BW? lower Zout?
Thanks Sunnyskuyguy :)
In the circuit given above, the inputs are already out of phase, so the above circuit is just a differential amplifier. I need to generate differential signals using BJT. Can we generate differential signals just by giving a single input.
Crustchow has given a circuit below that is perfect but now the standard one.
If you can help..
Thanks :)

- - - Updated - - -

Here's a simulation of a simple single transistor single-ended to differential converter.

View attachment 118048
Thanks crutschow :)

This circuit is perfect :)
I think there is some circuit with back to back BJTs (The standard one) to generate differential signals. If you could help?

- - - Updated - - -

If you can provide the circuit, it will be extremely helpful :)

Thanks in advance.
 

FvM

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In the circuit given above, the inputs are already out of phase, so the above circuit is just a differential amplifier. I need to generate differential signals using BJT. Can we generate differential signals just by giving a single input.
That's in fact possible and the shown differential amplifier ("long-tailed pair") is well suited for it.

I think it's time that you start to analyze circuits yourself, either by pencil and paer method, or by using a circuit simulator like LTspice. Only asking others to provide you the results won't bring you far.

Or read text books that do the circuit analysis, e.g. Analysis and Design of Analog Integrated Circuits by Grey, Hurst, Lewis, Meyer.
 

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Debdut

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Did the post I wrote not solve the problem?
 

SunnySkyguy

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Thanks Sunnyskuyguy :)
In the circuit given above, the inputs are already out of phase, so the above circuit is just a differential amplifier. I need to generate differential signals using BJT. Can we generate differential signals just by giving a single input.
Crustchow has given a circuit below that is perfect but now the standard one.
If you can help..
Thanks :)

- - - Updated - - -



Thanks crutschow :)

This circuit is perfect :)
I think there is some circuit with back to back BJTs (The standard one) to generate differential signals. If you could help?

- - - Updated - - -

If you can provide the circuit, it will be extremely helpful :)

Thanks in advance.
My answer may be either diff input or single ended input as long as they have the same DC bias. THis also has gain and no DC offset in the differential output. Using split supplies it possible to have null common mode offset to ground as well.
 

Warpspeed

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Can we generate differential signals just by giving a single input..
Yes.
Use the SunnySkyguy's circuit, and just drive one input, and ground the other.

This is the same "long tailed pair" circuit mentioned earlier in the thread by Betwixed.
 

Rahul Soni

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At the base of one BJT of the pair give dc bias to operate it in active region. At the base of the other BJT give the same dc bias using a resistor.
Ok..
Apply the signal, whose differential you want to create, to the base of the 2nd BJT using a capacitor.
How can i aplly two inputs at the base of second BJT?
 

SunnySkyguy

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  • YOu do not need to drive both inputs.
  • Just like an Op Amp, you can use either input
  • Connect unused input to Ground or other DC reference that matches what you want. Also match input bias Resistor to Gnd on both sides so input current does not create an offset voltage, ( same in Op Amps)
  • Output 1 is non inverting from Input 2
    or
  • Output 2 is non-inverting from Input 1
  • The other output is complementary or inverting.
  • This generic differential stage allows single to diff, diff to diff, single to single or diff to single, in terms of input to output.
  • For null collector voltage, Rc=0 with a split supply ensure input offset, Vio = 0
  • i.e. let DC Input 1= Input 2, i.e. same DC level,then apply signal to either with a reasonable low impedance source. ( much lower than input impedance, so as not to attenuate input.
  • Input impedance will be Zin ≈hFE*Re
  • Single ended gain Av = Rc/Re with no load RL=∞
  • otherwise gain, Av={RL//Rc /Re}
    ( Rc, RL in parallel at collector )

    Re controls gain with smaller values and may be two in series with one bypassed with large C above frequency(HPF)
to improve gain but reduce input bias current. Re can also be replaced with a current sink.
 
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