THAT2151 Datasheet, PDF (7/10 Pages) List of Unclassifed Manufacturers – IC Voltage-Controlled Amplifiers

Rev. 10/25/96
Page 7
Figure 10. 1kHz THD+Noise Vs. Input, -15 dB Gain
Figure 11. 1kHz THD+Noise Vs. Level, 0 dB Gain
Figure 12. 1kHz THD+Noise Vs. Input, +15 dB Gain
Applications
Input
As mentioned above, input and output signals are
currents, not voltages. While this often causes some
conceptual difficulty for designers first exposed to this
convention, the current input/output mode provides
great flexibility in application.
The input pin (pin 1) is a virtual ground with nega-
tive feedback provided internally (see Figure 6, Page 4).
The input resistor (shown as 20 kΩ in Figure 3, Page 3)
should be scaled to convert the available ac input volt-
age to a current within the linear range of the device.
(Peak input currents should be kept under 1 mA for
best distortion performance.) An additional consider-
ation is stability: the internal op amp is intended for
operation with source impedances of less than 30 kΩ
at high frequencies. For most audio applications, this
will present no problem.
the open-loop gain naturally falls off at high frequen-
cies, asking for too much gain will lead to increased
high-frequency distortion. For best results, this resis-
tor should be kept to 10 kΩ or above. Distortion vs. fre-
quency for a 1 V signal at 0 dB gain with a 20 kΩ input
resistor is plotted in Figure 13.
The quiescent dc voltage level at the input is ap-
proximately +10 mV. As mentioned above, any dc input
currents will cause dc signals in the output which will
be modulated by gain, causing audible thump. There-
The choice of input resistor has an additional, sub-
tle effect on distortion. Since the feedback impedances
around the internal opamp (essentially Q1/D1 and
Q3/D3) are fixed, low values for the input resistor will
require more closed-loop gain from the opamp. Since
Figure 13. THD Vs. Frequency, 0 dB Gain
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