|TL026CD||DIFFERENTIAL HIGH-FREQUENCY AMPLIFIER WITH AGC|
|TL026CD Datasheet PDF : 13 Pages |
DIFFERENTIAL HIGH-FREQUENCY AMPLIFIER WITH AGC
SLFS007A – JUNE 1985 – REVISED JULY 1990
Amplifier A2 inverts V1 producing a positive AGC signal voltage. Therefore, the input voltage to the TL026C
AGC pin consists of an AGC signal equal to:
and a dc voltage derived from Vref, defined as the quiescent value of VAGC.
+ ) VAGC(q)
For the initial resistor calculations, Vref is assumed to be typically 1.4 V making quiescent VAGC approximately
1.22 V (VAGC(q) = Vref – 180 mV). This voltage allows the TL026C to operate at maximum gain under no-signal
and low-signal conditions. In addition, with Vref used as both internal and external reference, its variation from
device to device automatically adjusts the overall bias and makes AGC operation essentially independent of
the absolute value of Vref. The resistor divider needs to be calculated only once and is valid for the full tolerance
output voltage limits (see Figures 6 and 7)
The output voltage level desired must fall within the following limits:
1. Because the data sheet minimum output swing is 3 V peak-to-peak using a 2-kΩ load resistor, the
user-selected design limit for the peak output swing should not exceed 1.5 V.
2. The voltage drop of the rectifying diode determines the lower voltage limit. When a silicon diode is
used, this voltage is approximately 0.7 V. The output voltage VO must have sufficient amplitude to
exceed the rectifying diode drop. Aschottky diode can be used to reduce the VO level required.
gain calculations for a peak output voltage of 1 V
A peak output voltage of 1 V was chosen for gain calculations because it is approximately midway between the
limits of conditions 1 and 2 in the preceding paragraph.
Using equation 3 (VC = VOP – Vd), VC is calculated as follows:
VC = 1 V – 0.7 V
VC = 0.3 V
Therefore, the gain of A1 must produce a voltage V1 that is equal to or greater than the total change in VAGC
for maximum TL026C gain change.
With a total change in VAGC of 360 mV and using equation 4, the calculation is as follows:
* + + + + V1 DVAGC
If R1 is 10 kΩ, R2 is 1.2 time R1 or 12 kΩ.
Since the output voltage for this circuit must be between 0.85 V and 1.3 V, the component values in
Figures 6 and 7 provide a nominal 1-V peak output limit. This limit is the best choice to allow for temperature
variations of the diode and minimum output voltage specification.
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