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Bipolar 4R CE Amplifier
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Theory
=======

See notes and textbook on 4R biasing circuit.

Simulation
============

See homework on simulating 4R bias circuits and amplifiers. 
Simulation can be done with either Multisim or Pspice.
    

Measurement
============

    
Circuit Configuration
---------------------
:ref:`fig-4r-schematic` is shown below: 

.. _fig-4r-schematic:
.. figure:: images/4rschematic.png
   :scale: 100 %
   :alt: 4R bias amplifier schematic 
   :align: center
   
   The schematic of a bipolar transistor amplifier using a 4R bias circuit.

We can immediately recognize the 4R biasing circuit used. 
Note that we have two Re's, 
Re1 and Re2, Re1 is for negative feedback in "ac" signal. 
Re2 is for negative feedback in DC bias.
Exactly how to choose (IC,VCE) 
will take more discussions which will be
covered in *Analog Electronics*. 
For now, let us consider that we have been given a (IC, VCE) bias point. 
We have designed a biasing circuit to produce the required (IC, VCE) using
a Si bipolar transistor, 2N3904 in this experiment.


We have added 3 capacitors that are going to block DC 
and pass AC. Of course, capacitor's 
impedance depends on frequency.
The :ref:`fig-4r-photo` is shown below:

.. _fig-4r-photo:
.. figure:: images/4r-board-photo.png
   :scale: 100 %
   :alt: 4R bias amplifier photo 
   :align: center
   
   Photo of a bipolar transistor amplifier using 4R bias

    
    

Elvis Wiring
------------
In the photo, 
the AC input voltage is wired to both AI0 and SCOPE CH0. 
The AC output voltage is wired to both AI1 and SCOPE CH1. 
The AI inputs have a sampling rate limit of 1.25 Ms/second, 
The SCOPE inputs, however, have a much higher sampling rate
of 100 Ms/second.


The function generator supplies the AC input.
We can now vary the input amplitude and frequency and 
observe the input and output waveforms on the scope channels.


.. _fig-4r-func-generator:
.. figure:: images/4r-func-generator.png
   :scale: 100 %
   :alt: 4R bias function generator
   :align: center
   
   function generator setting

Sample Waveforms
------------------
Typical :ref:`fig-4r-vout-vin` are shown below for 
a small input voltage with a peak to peak voltage of 
100 mV.    Note the scale difference for the two channels.
The input and output show a 180 degree phase difference,
as expected from transistor's natural inverting
property.
The amplifier gain is approximately 25, or 
approximately 30 dB if you do the conversion math.

.. _fig-4r-vout-vin:
.. figure:: images/4r-vout-vin.png
   :scale: 100 %
   :alt: 4R bias vout vin
   :align: center
   
   input and output waveforms
    
Next, set frequency to 20kHz, Vpp of input
to 0.07V. You should see signs of distortion
in the output waveform, as shown below:

.. _fig-4r-vin-0p07v:
.. figure:: images/4r-vin-0p07v.png
   :scale: 100 %
   :alt: 4R bias distortion
   :align: center
   
   input and output waveforms for a 20 kHz, Vpp=0.07V input

Now you can vary amplitude and frequency to see how the 
amplifier output changes. In the live demo I gave in class, you have seen:

* Gain compression at high input power
* Bode plots, or the magnitude of 
  phase of the voltage gain as a function of frequency
* Output waveform distortion at high input power
* Output waveform for different frequencies



   
.. note::

    Keep in mind that as you adjust the input amplitude, 
    your scope settings will need to be adjusted accordingly.
    Otherwise, you can see artifical clipping of your 
    signals, simply because the voltage values are out of range
    for the settings of your scope.