AN145 ne5517/a transconductance amplifier applications 1988 dec integrated circuits
philips semiconductorscs application note AN145 ne5517/a transconductance amplifier applications 2 1988 dec description the philips semiconductors ne5517 is a truly versatile dual operational transconductance amplifier . in plain language, it is a voltagetocurrent converter governed by the transconductance g m , which is equivalent to i out /v in . the g m is increased or decreased linearly by varying the amplifier bias current (i abc ) through an external pin (see figure 1). from the proper use of the i abc pin, many control circuits can be realized. for more insight into the way the part operates, the transconductance can be thought of as gain and is governed by the following equation: g m i out v in i abcq 2kt (1) where the transconductance is dependent on the constant kt/q (which is 26mv at 25 c, and i abc (which is controlled by the user). t o make the device more universal and adaptable for many functions, two impedance buf fers for voltage output applications are also included with the amps so that the part can be used as a programmable operational amplifier . linearizing diodes provide another useful option. these should be applied when large input voltages or wide temperature variations are encountered. t o show the significance of the diodes, compare the dif ference between equation 1 without diodes and equation 2 with diodes: i out v in 2i abc r in i d (2) for i in greater than i d 2 here, it can be seen that the transconductance is not temperature dependent. r in is the signal input resistance and i in is the signal current. i in must not exceed half the diode current (i d , nominally 1ma). the diode current is set by a resistor tied to +v cc . a graph showing the output distortion improvement versus dif ferential input voltage when using the diodes is shown in figure 2. an advantage that the ne5517 has over similar devices is a special biasing network between the amplifier and output impedance buf fers. this network eliminates output of fset current changes with a sudden change in the bias current (i abc ). this is particularly important in audio applications where an audible of fset would be produced. applications an application employing both amplifiers and buf fers internal to the ne5517 is the adjustable trianglesquare wave generator shown in figure 3. the center oscillating frequency is set by the capacitor c at the output of amplifier a. the output amplitude is set by the resistor r connected between the noninverting inputs, amplifier b output, buffer b input and ground. the oscillating frequency is varied by changing v c , which in turn controls the amplifier bias current (i abc1 ). if a positive voltage is applied to v c , the center frequency will increase linearly with increasing voltage. if a negative is applied, the center frequency will decrease linearly with increasing negative voltage. this makes a very good programmable oscillator with variable amplitude. by using a large value capacitor and negative control voltage, oscilla - tions in the fractions of hertz can be realized; a small capacitor and positive control voltage will give frequencies up to 500khz. graphs showing the linearity of control voltage versus frequency for dif ferent capacitor values are shown in figure 4. pertinent calculations are: f c i abc1 2(c) (i abc2 ) (r) where : f c center frequency i abc1 oscillator control current i abc2 amplitude control current r amplitude control resistor c oscillator control capacitor also : amplitude (i abc2 ) (r) another very useful application is to use the ne5517 as a digitallyprogrammable amplifier . the entire circuit is shown in figure 5. the circuit consists of a philips semiconductors microprocessorcompatible dac, a transistor array, and the ne5517 configured as a voltagecontrolled amplifier . this arrangement can also be used with the vco explained earlier to program its oscillating frequency. the pertinent equations governing this application are as follows: a v v out v in bw(10) 256 x i dacmax x q x r l 2 x kt where : bw(10) binary word decimal i dacmax maximum dac output current (1ma) r l load resistance (30k) q kt 38.5 at 25 o c also: i dacmax 2 x v ref r ref 2 x 5k 10k 1ma where : v ref supplied by dac (5v) r ref referenced resistor (10k ) the i dac max of 1ma is used to keep the transconductance within the linear range. the current mirror matches the current flow into the dac and supplies the same amount to the 5517 control pin. using a current output dac is much faster than using a voltage output device to control the part. (if speed is not important, this can be done and the current mirror can be replaced with a resistor .) also, the gain equation pertains to the signal after the input divider .
philips semiconductorscs application specification AN145 ne5517/a transconductance amplifier applications 1988 dec 3 sl00883 figure 1. pin designation and functional diagram sl00884 figure 2. output distortion vs input voltage showing benefit of diodes
philips semiconductorscs application specification AN145 ne5517/a transconductance amplifier applications 1988 dec 4 sl00885 figure 3. triangle-square wave generator note: v c below 13.6v or above +30v will cause distortion. changing the c value will vary the frequency range. a. b. c. sl00886 figure 4. control voltage (v c ) vs frequency data
philips semiconductorscs application specification AN145 ne5517/a transconductance amplifier applications 1988 dec 5 sl00887 figure 5. digitally-programmable amplifier
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