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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)