The
LM3900 is a 14-pin DIP containing four identical op-amps, each with inverting
and noninverting inputs and an output. However, these op-amps are very different
from the usual op-amp, and must be used in a completely different way. The usual
op-amp responds to a differential voltage at its inputs, but the LM3900 responds
to a differential current. Instead of a differential amplifier, the input
stage is the current-differencing circuit shown at the right. The current mirror
at the noninverting input subtracts the current at that input, I+, from the
current at the inverting input, I-, to form the difference current I- - I+ that
is furnished to the amplifier with overall gain of 70 dB. If I- is greater than
I+, the output saturates low, and if I- is less, the output saturates high.
Feedback from the output to the inverting input acts to reduce the difference
current, which in normal operation is very small. This is just like the usual
op-amp, except with current instead of voltage. The output, however, is a
voltage as with the usual op-amp.
Another difference that is immediately obvious is that the inputs are one diode
drop, about 0.55 V in the LM3900, above ground, and vary little from that
voltage. There is no common-mode voltage range at all! It has been replaced by a
common-mode current range by the current differencing. Equal currents supplied
to the two inputs are not amplified. The LM3900 is specially useful when only a
single power supply is available. Its output swings from very near ground to a
diode drop below the positive supply. In these experiments, we'll use a +12 V
supply. The LM3900 can use from 4 to 36 V total supply voltage. There is one set
of power connections for the four amplifiers, but they are otherwise
independent. Connections for the LM3900 are shown at the left.
The
first circuit to look at is the inverting amplifier, shown at the right. Note
the symbol for the Norton op-amp, that has a current source between the inputs,
and an arrow on the noninverting input. This is an AC amplifier, with coupling
capacitors at input and output so there is no worry about DC bias levels, which
can be chosen as necessary. The input and feedback resistors are as with the
familiar circuit, and the gain in this case should be -2. The difference is the
39k resistor at the noninverting input. It supplies a current (12 - 0.55)/39 mA
to this input, and the output endeavors to supply an equal current to the
inverting input, which requires a DC output voltage of (20/39)(12 - 0.55) + 0.55
= 6.4 V, a convenient bias. The bias is set by the current I+, not by a voltage
divider, as it would be with the usual op-amp. The LM3900 is very convenient for
AC amplifiers. When you test this circuit, the scope traces for input and output
can be superimposed with proper setting of the gain, and the inversion is
obvious.
A
unity-gain buffer is shown at the left. It looks like the same circuit for the
usual op-amp, but here the resistors are necessary. The input resistor turns the
applied voltage into a current, while the feedback resistor causes the output to
rise to a value supplying an equal current to the inverting input. The circuit
is not as precise as the usual one, with small shifts due to the diode drops at
the input, but the circuit is useful and furnishes a reasonable input resistance
of 1M. The extremely high input resistance of the voltage or FET-input op-amp is
not obtained, however.
The
unity-gain buffer is a special case of the noninverting amplifier, shown at the
right with biasing by means of a voltage divider. The divider supplies a current
into what is effectively a unity-gain buffer to reproduce its voltage at the
output. This amplifier has a gain of +10, of course, or 20 dB. Again, the high
input impedance of the voltage op-amp is not obtained.
Another
biasing scheme for the inverting amplifier is shown at the left. It relies on
the diode drop at the inverting input to supply a current proportional to
Vbe, which is then amplified by the feedback resistor. In this case,
the bias is about 10 x 0.55 = 5.5V. Actual measurement gave 5.7 V. Here, the
current supplied by the output is returned to the inverting input, where it
flows out the 120k bias resistor, so the bias is positive. In this case, the
bias does not depend on the supply voltage.
A
voltage regulator with a Zener in the feedback loop is shown at the right. The
510Ω resistor supplies a bias current of 0.55/0.51 = 1 mA to the Zener to reduce
noise and improve stability. This voltage is added to the Zener voltage at the
output. A 4.2 V Zener (1N5230) gave 4.76 V, a 2.3 V Zener (1N5226) gave 2.84 V.
A pass transistor can be added at the output for additional current capacity
(the LM3900 can be depended on only for about 20 mA when sinking current).
A
relaxation oscillator is shown at the left. With the values shown, this circuit
oscillated at 3840 Hz, the output swinging from near zero to 11.3 V. The slew
rate of the LM3900 was evident in the output waveform. To understand this
circuit, assume for simplicity that the output swings from 0 to the supply
voltage V. When the output is 0, a current I+ = V/10 μA is supplied to the
noninverting input. When the output is V, a larger current I+ = V/5 is supplied.
These are the trip levels expressed in currents. When the capacitor is
discharged, I- = 0, so the output is high, and the trip level is 3(V/5) = 0.6V.
The capacitor charges until its voltage reaches this level, but then the ouput
goes low and it begins to discharge again. The lower trip level is 3(V/10) =
0.3V. When the capacitor voltage reaches this level, the ouput again goes high,
and the cycle repeats. For a 12V supply, these levels are about 7.2 and 3.6
volts. This is a very rough calculation, so we expect only general agreement.
The measured values (from the scope) were 3.0 V and 8.4 V, and the capacitor
voltage waveform was quite rounded on top and bottom--in fact, it would be a
passable sine wave if you were not too particular. Buffered, this would be an
easy way to get a sine wave.
The
relaxation oscillator makes use of a Schmitt trigger, with positive feedback. An
inverting Schmitt trigger is shown at the right. The input and supply voltages
can be exchanged to give a noninverting circuit. The output goes from high to
low when the input reaches 7 V, and from low to high when it goes below 6 V,
with a hysteresis of 1 V. The hysteresis can be displayed directly on the scope,
using the X-Y display. Connect Ch 1 to the input voltage, and Ch 2 to the
output. A 100 Hz triangle wave from a function generator makes a suitable input.
Alternatively, use 0.5 Hz and watch the dot move. This is a very graphic
illustration of Schmitt trigger operation.
The Norton op-amp can also be used to make active filters and oscillators; see the National Semiconductors application note AN-72 for details.
The National Semiconductors application note AN-72 of Sept. 1972, revised June 1986, is a good introduction to the LM3900, with a great variety of applications.
Composed by J. B. Calvert
Created 10 August 2001
Last
revised 11 August 2001