Download chapter sb basic applications of: operational transconductance

CHAPTER SB
BASIC APPLICATIONS OF:
OPERATIONAL TRANSCONDUCTANCE AMPLIFIERS
CONTENTS:
Introduc tion
Basic Setup of the CA3080
Typical Gain Controlled Circuit
Signal Switching Methods
Current Switching Methods
INTRODUCTION
The Operational Transconductance Amplifier (or its functional equivalent) is the
single most important component for electronic music devices.
Nearly every device
requires a wide range controlled gain block.
The OTA in common use is the RCA type
CA3080.
It has a differential input stage that is driven by an externally programmable
bias current that controls the gain in a manner similar to the two quadrant transconductance multiplier (see chapter 5c on VGA design). There are no resistors on the chip instead a series of transistor current mirrors are used. The output is a current rather
than a voltage.
The fact that the OTA is current controlled is extremely important since it can be
directly coupled to a wide range exponential current source.
[It would be more convenient if the OTA were controlled by a current drawn out of it as it could be coupled
directly to NPN exponential sources. The OTA requires a PNP exponential source or a
current mirror. If a PNP source is used, an inverting stage is required to make control
voltage response conventional - higher control voltages represent an increase in the
controlled parameter.]
As a controlled gain stage, the OTA forms the basis of VGA's and VCF's.
It can
also be used as a signal switch for devices like sample-and-holds.
With a saturated
input, it can be used as a current switch useful for VCO's.
3b (1)
BASIC SETUP OF THE CA3080
The 3080, being a transconductance device has a transfer function I0ut/Ein = §m> where
gm is the transconductance.
Since the 3080 has a standard differential two transistor
input stage, the transconductance is given by [see chapter 5c]:
This gives the basic input-output relationship:
I
out
t
ma
=19
2
Win
t
*
ma
volts
BASIC CA3080 EQUATION
I ABC is the Amplifier bias current and ranges from about 0.5 microamps to 0.5 ma*. The IABC
can be supplied by a current source, or for a simple setup by a resistor between pin 5 and
some voltage between -14.4 volts and +15 volts.
Pin 5 of the OTA is approximately one diode drop
above the negative supply. With such a setup,
3080 Base and
Z
ABC ls given by:
Pin 5 voltage
introl
'ABC ' <Vcontrol+1
This suggests a good test setup consisting of a
27k resistor connected between pin 5 and the
wiper of a pot connected between +15 and -15
as indicated at the right.
The OTA is often used "open loop" and
no compensation is required. When compensation
is required, it has to be approached as a worse
case problem, as the actual compensation depends
on IABC an<^ ^n most electronic music devices, IABC
is a variable.
The compensation for unity gain
is shown at the right. It is also extremely
fortunate that in at least two cases where a closed
loop is used (sample and hold, voltage-controlled
integrator) the compensation is a part of the
circuit anyway.
TEST
SETUP
3080
308pX^
UNITY GAIN
COMPENSATION
I
TYPICAL GAIN CONTROLLED CIRCUIT
A typical gain controlled circuit is shown at the top of the next page. The circuit
can be thought of as the basis of a VGA.
The following is an outline of the design:
(1)
The circuit is open loop (no feedback) so no compensation is required.
(2)
We select input signals in the range of ±5 volts.
The actual OTA input stage
* Note that the CA3080 can stand IABC values UP to 2ma. There are at least two good
reasons for not going this high: (1) The device looses linearity, and (2) It can start
to warm up and the 3080 is famous for the ease with which it can be coaxed into thermal
runaway. Considerable care should be used to make sure the current to pin 5 is not
allowed to rise too high in a test setup.
A direct short to ground of this pin, for
example , will destroy the device .
3b (2)
•
'°0*
<WWv
±10 mv
OUT
IN ±5
BASIC CA3080 CONTROLLED
GAIN CIRCUIT
Is the ordinary two transistor differential amplifier. Thus, for linearity the inputs
should be limited to 10 mV [see chapter 5c].
Therefore, a 500:1 voltage divider
(100k:200S2) is used on the input.
(3) If DC balance is critical, a balance stage as indicated should be used on
the + input. If this is not necessary, the + input can be simply grounded.
(4) At the maximum value of control current (I,RC) that we choose (say 0.5 ma)
we want the circuit to have unity gain. We thus calculate using the basic equation
for the 3080:
I
(max) = 19.2-(0.5 ma)-(0.01 volts)
out
= 0.096 ma
When the current to voltage converter A2 is attached, we want the maximum output current
to drive the output voltage to ±5. This gives R = 5/0.000096 = 51k
(5) The signal was supplied to the - input of Al because the
current to voltage converter A2 is also inverting.
In open loop
circuits, the only difference between the inputs to the OTA is the
phase. [It may be helpful here to think of the OTA as an op-amp
comparator with limited gain. Reversing the inputs changes the
output phase by 180°, but the amplifier remains in the linear
region.]
There are two changes that will give an overall inversion
to the output: (a) The + and - inputs can be reversed, and (b) The
current to voltage converter at the right can be used.
(6) The control current can be supplied for a test setup
by a 56k resistor connected to a pot wiper as suggested above.
The 56k resistor across approximately 30 volts gives the max.
current of about 0.5 ma for I AT i r «
NON-INVERTING
CURRENT TO VOLTAGE
CONVERTER
BMC
SIGNAL SWITCHING METHODS
The OTA can be used in two "switching" modes. In the first mode, an audio signal
at the input is switched by applying or removing the bias current. The simplest method
of doing this is to have the current to the control pin be supplied by a resistor that
is connected to a voltage that switches between the supply limits.
A second method
uses a transistor to drive pin 5 so that a 0 to +5 logic level can be used to control
the switching.
The CA3080 used in this way is an analog switch.
Shown also below
is the setup for a sample and hold circuit.
3b (3)
3080
15k
Switching
Control Current
10k
+5 = ON
0 = OFF
BASIC SAMPLE-AND-HOLD
CURRENT SWITCHING METHODS
The second switching mode is one where the input voltage differential is saturated.
This has the effect of transferring the input bias current to the output in a direction
determined by the sense of the input voltage. This can best be seen by considering the
current mirror structure of the OTA as shown below.
When the input voltage differential
reaches about 40 mv or higher, most of the bias current passes through one of the transistors while very little passes through the other.
The attached current mirrors thus
either receive the bias current or no current at all.
The OTA in this state is a voltage reversible current source useful for VCO circuits.
CURRENT MIRROR STRUCTURE
STRUCTURE OF OTA
-\5
foe*
TWO STATES OF CURRENT SWITCHING OTA
3b (4)