ABOUT OPERATIONAL AMPLIFIERS
By Eddie Ciletti for Studio
Maintenance Series
In order to have voltage gain, the most basic,
single-stage amplifier – vacuum tube or solid state – will invert the polarity
as part of the process. It is easier to understand voltage gain (a mic
preamp) than it is current gain (a power amp), the latter having the ability
to do work, to break a sweat, to get hot. (Do ya get it?)
A transformer can manipulate voltage between primary
and secondary but not without an inverse change of current. Step the voltage
up, for example, and the current capacity comes down and vice versa. The
power formula P= VI (or E*I) shows this relationship quite clearly. With
120 volts on a primary that is capable of carrying 1-amp of current, the
power at the primary is 120 watts. Step down to 12-volts on the secondary
(a 10:1 ratio) and it’s quite possible for the secondary to have a 10-amp
capacity. Again, the power, 12-v times 10-A = 120 watts.
Most power transformers are not quite that efficient
and most amplifiers are even less efficient. Power amps also require a
power supply, but aside from that, a power amp can convert impedance from
high-to-low and essentially allow any voltage of any impedance to drive
a low impedance load, such as a speaker, without the voltage and current
trade-off.
Now what you might think of as a power amplifier
is not limited to driving speakers. It is simply a matter of relativity.
A condenser mic requires an impedance converter just to convert delicate
changes in sound pressure to microphone level. Whatever is inside a microphone
constitutes both voltage and current gain. That said, we will initially
look at opamps as voltage amplifiers even though they are simultaneously
behaving as power amps.
The operational amplifier (Op Amp) was originally
developed to perform arithmetic "operations" for an analog computer,
hence the name. For example, voltage gain is multiplication. A summing
amplifier (like a mix buss) is simple addition. A differential amplifier
performs subtraction.
All opamps have at least three signal connections
- two inputs and an output - one input flips the incoming signal polarity
by 180 degrees (inverting = "-") while the other input does nothing to
the signal polarity (non-inverting = "+").
Most opamps operate on bipolar power, typically plus
15-volts and minus 15-volts. This allows nearly a 30-volt peak to peak
signal to develop. It is just as easy to run the same opamp on a 9-volt
battery, the amplifier must simply be biased at half the supply voltage,
4.5-volts in this case. Can you draw a circuit to accomplish this goal?
The OPAMP graphics page has five configurations that
you should be able to draw from memory. All of these amplifiers appear
in some form on the LA-3 student kit schematic, with which you should also
be familiar enough to be able to draw from memory. There are several more
opamp configurations, the side-chain of a compressor-limiter being examples
of "application specific" circuits. You should at least be able to recognize
those that are in the LA-3 kit. There is one packaging variation between
the IC opamps on the graphic page versus those in the LA-3 kit. Can you
determine that difference?
SUBTRACTION
The Input Amplifier circuit used in the LA-3
kit must accept a balanced line-level signal – two signals of opposite
polarity. If these two signals were brought up on a pair of faders, they
would cancel out. But a differential amplifier does just that, amplify
by subtracting the difference. With all resistors being equal, a 1-volt
signal at each input – one being of opposite polarity - the equation for
what happens next is V2
- V1 * (R3
/ R1) or 1 – (-1)
* (10k-W /10k-W
) = 2 * 1 = 2, a gain of 2, not zero (not cancellation).
Conversely, if the two signals were identical, including
polarity (as would be the case with induced noise) the difference amp would
subtract one from the other and get ZIP, a.k.a. "squat!" The induced noise
is also known as the Common Mode signal and the amplifier’s ability to
discriminate between desirable signals and noise is known as the Common
Mode Rejection Ratio or CMRR.
VOLTAGE FOLLOWER
At the top left of the OPAMP page is a unity gain
non-inverting amplifier known as a Voltage Follower. There are two in the
LA-3 kit, both isolate or "buffer" the optical attenuators (one for AC
and the other for DC) from the circuitry that follows. For example, when
the photo resistor is 82k-W
, the divider cuts the signal in half (that’s –6dB). IF the junction of
these two resistors had to drive a pair of potentiometers – the LEVEL and
Gain Reduction pots for example – the level at the divider would significantly
change. (The combined pot values (two 10k-W
resistors in parallel = 5k-W
). Surely you can see that this would be a problem – can you draw and explain
why – be thankful for the Voltage Follower.
NOTE: Opamps in general, and this circuit
in particular, have a very high input impedance - so as to not load the
divider - and low output impedance - so as to not be bothered by driving
two pots.
INVERTING AMP (Addition)
At top right is an amplifier that serves several
functions. In recording consoles it is the summing buss for the main stereo
mix, aux sends and groups. When R1
(the input resistor) and Rf
(the feed back resistor) are equal, the signal Vin
equals the output signal Vout
–
the gain is unity. In the LA-3kit, opamp 2a is the make-up gain amplifier
that follows the Level pot, where R1
=
10k-W andRf
=
110k-W . Can you calculate
the gain for this amplifier? This amplifier drive the RING of the TRS output
connector.
Following the make-up gain amplifier is another inverting
amplifier, this time the relationship between input and feedback resistors
is the same, a ratio of 1. The purpose of this amp is simply to invert
the polarity of the signal to drive the TIP of the output TRS connector.
NON-INVERTING AMPLIFIER
The formula for the non-inverting amp pretty much
says it all – it’s one PLUS the relationship between the voltage divider
R1
andRf.
If
these two resistors are identical, the signal from Vout
is divided in half to the inverting input. So, the formula 1 + Rf
/R1
=
2 and if you haven’t figured it out, a gain of 2 means that
Vout
is
DOUBLE Vin and uh, for
the umpteenth time, that’s a gain of, well, I hope you guessed
6dB!!!
The EL panel has a fairly wide dynamic range; they
can be connected directly to the 120-volt RMS power line. It is a high
impedance device. It will show visible light with only 80-volts RMS. An
opamp running from a bipolar 15-volt supply can deliver a nearly 30-volt
P-to-P voltage swing, not enough to excite the EL panel. However, with
a step up transformer, 30-volts AC can easily be transformed to 80-volts.
The current required to drive the step-up transformer
is significant, so the non-inverting amp used to drive the EL panel is
"buffered" by an external pair of transistors to increase the current drive
/ power capacity. The devices have about 4-times the surface area of the
opamp and so are capable of dissipating more heat.
BALANCED OUT
There are several ways to make balanced input and
output amplifiers, the differential output amp on the "graphics" page is
similar to the one we used for the LA-3 kit with one exception – this amplifier
assumes the incoming signal, Vin
is at nominal level. The LA-3 "RING" output amp was also the make-up gain
amplifier. The version shown on the graphics page has two cascaded unity-gain
inverting amplifiers.