STYLE=" margin-top: 12pt;" CLASS="Normal">Because you will use a
number of transistors in this lab, you will construct your circuits
using packages that contain multiple transistors.

The first package, the
LM308N consists of a number of NPN transistors. The second package,
the MPQ3906 consists of a number of PNP transistors. Data sheets for
both of these parts are appended at the end of this lab. The diagrams
we provide will not have pin numbers on them part of your work will
be to decide which transistors to use for various purposes.

We will start with some
important building blocks and we will make an intermediate device
sort of an AC coupled OP-AMP. This will be the work for the first week.
In the second week, we will expand this circuit to build the multiplier.

Week One Building Blocks

1. Current Source

The first circuit we need
to build is a current source to provide approximately 1 mA of current.
We will use a current mirror design.

Enter the following schematic
intro PSPICE (you can use Q2N3904 transistors) and run a simulation
for one cycle of the output load. Arrange the output plot so that both
the voltage and current are on the screen, and print it out. Explain
the non-ideal behaviors you notice in the output current as a function
of output voltage.

Now go ahead a build the
circuit using the LM3086N (you probably want to use one of the matched
pairs on the chip). Connect your signal generator to a 1 K resistor
as shown below, and use the scope to measure both sides of the resistor.
Use the subtract math function for both channels of the scope, and
see if the non-ideal behavior of the circuit matches your expectations
(note the signal generator will only be able to generate about +/-
10V.

2. Degeneration

Now add another resistor.
Simulate and build the following circuit and explain the voltage waveform
you observe at the output of the current source (the green probe) as
a function of the right hand voltage V1 (red probe). Try reducing the
amplitude of V1 Explain how V1 steers the current to the left or the
right side:

Next, add two transistors
and two resistors to the circuit, and see what the results are, both
in simulation and in the real circuit. Explain the action of the upper
transistors in both steering the current based on the base voltages
and in delivering the current up to the collectors. Note that the amplitude
of the voltage source below is 1 Volt. Try turning it up to 2 Volts.
Explain what happens, and why.

3. A Mirror Above

Lets say we wanted to
use the above circuit as an op-amp, but wanted a single-ended (not differential)
output. We can use a current mirror on the top end to do this. Simulate
and built the following circuit. Note that the top transistors are PNP
(you can use a 2N3906 for this in simulation). Also note that we AC-couple
the output, and that the input amplitude is reduced and the frequency
is increased.

After simulating and building
this circuit, explain how it works (especially how the AC coupled 10k
load works) and why the gain is less than 10X.

Week 2 The MULTIPLIER

Below is a summary of the
history of Translinear Design by Barry Gilbert. It is fun and educational
to read.

The Beginning
of Translinear Circuit Design by Barry Gilbert

 

I</span>n the autumn of 1954, I met my lifetime
companion and partner, whom we'll identify just by the initials "BJT,"
at a research lab in the south of England. After a decade of deepening
familiarity and joyous discovery together, we moved to Oregon, to continue
our adventures at Tektronix. This company, dedicated to fashioning the
world's finest oscilloscopes, took the unusual step in 1965 of establishing
an integrated circuit production line, to support future instrumentation.

 

Although the wafers were only one inch in diameter, this "move
to monolithic" was to herald a new life for BJT, and was unquestionably
the most significant milestone in my life. This was a time of charming
naiveté, long before microprocessors and memories made an appearance.
IC masks were generated by taking an Exacto knife to "Rubies"
and cutting them practically straight from hand-drawn, slide-rule-designed
schematics. SPICE was brewing in Berkeley, but not quite ready for serving.

 

At Tektronix, my job was to design the new 7000-series oscilloscope,
using custom ICs to address the growing sophistication and bandwidth
of these instruments. However, I had enormous latitude to experiment
in this new medium, which allowed me to develop many interesting and
curious circuit cells. Equally exciting were novel semiconductor devices
exploiting "super-integration." Unlike conventional node-by-node
circuit development, where the physical extent of a transistor had no
fundamental bearing on the function, these structures exploited the
juxtaposition of numerous transistor fragments-bits of NPNs and lateral
PNPs-to elicit specific and practically useful functions, both analog
and logical. I2L was one later development of these ideas. 

Meanwhile, analog design was taking the first tentative steps to explore
new possibilities presented by the unique properties of monolithic fabrication,
namely: (1) the close matching and (2) isothermal operation of (3) many
essentially identical BJTs. Design in this medium was still in its infancy.
Only a handful of basic cells were widely known and used. Two youngsters
were especially promising: the differential pair and the current mirror.
An inevitable coalition was only steps away.

 

Each of these cells comprised only two transistors. But what a wealth
of possibilities they afforded! How predictable their mathematics, over
huge spans of collector current! What made the "diff pair"
so intriguing was the possibility of using it as an analog multiplier
by applying a voltage-mode signal across its base nodes and varying
the "tail" current (the common emitter bias) to alter the
magnitude of the differential collector current. As it happened, the
7000-series needed such a multiplier to provide a variable gain function
in circuitry buried deep in the system, which could be controlled at
a distant location (the front panel). Its bandwidth needed to extend
from DC to several hundred megahertz. 

The diff pair multiplier was well-suited to this challenge, except for
one problem: the linearity of the transconductance from the base nodes
to the output was poor (the nonlinear tanh function). On the other hand,
the current mirror was basically a pure current-mode circuit with good
linearity over a huge range of currents; and, as a signal processing
element, it exhibited a bandwidth close to the fT of the transistors,
which, in Tek's first-generation process, peaked at about 600 MHz.

 

So, the stage was set, but the plot began with a question: How could
the variable-gain aspect of the diff pair be combined with the linearity
of the current mirror? The felicitous answer sprang, as is so often
the case, by toying with "What If?" - the most potent path
to invention.

 

What if we placed two current mirrors side-by-side, with their output-side
transistors facing inward, and all the emitters grounded? In this topology,
their functions are completely independent. Now, what if we take that
Exacto knife and cut loose the lines to the two inner emitters from
their ground ties? What happens if we then rejoin these two emitters
at an independent node? The inner transistors have just become a simple
differential pair, but driven now from the difference voltage across
the outer, diode-connected transistors of the remaining fragments of
the mirrors. What if we now provide a variable current to that new center
node, to bias this fledgling diff pair? Bingo. 

When the excitement of discovery had abated, and pencil was placed more
thoughtfully to paper, it transpired that, with this simple bit of do-it-yourself
surgery, the four transistors had suddenly become something radically
different. We had a new entity: a purely current-mode multiplier cell
that operates in two-quadrants of the algebraic plane. Adding a second
similarly biased diff pair, their bases driven in parallel with the
first, but whose collectors are cross-connected to the output, created
the four-quadrant sub-nanosecond multiplier described in the paper.

 

That was only the beginning of translinear circuit design, which quickly
blossomed into dozens of fundamentally new cells, and which continues
to bear fresh fruit to this day.

Now consider the following circuit:

Build this circuit using the packaged transistors and a TL084 op-amp.
Using two signal generators, see what the circuit produces as output.