TL026C

DIFFERENTIAL HIGH-FREQUENCY AMPLIFIER WITH AGC

SLFS007A – JUNE 1985 – REVISED JULY 1990

5

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

gain characteristics

Figure 5 shows the differential voltage amplification versus the differential gain-control voltage (VAGC – Vref).

VAGC is the absolute voltage applied to the AGC input and Vref is the dc voltage at the REF OUT output. As VAGC

increases with respect to Vref, the TL026C gain changes from maximum to minimum. As shown in Figure 5 for

example, VAGC would have to vary from approximately 180 mV less than Vref to approximately 180 mV greater

than Vref to change the gain from maximum to minimum. The total signal change in VAGC is defined by the

following equation.

∆VAGC = Vref + 180 mV – (Vref – 180 mV)

∆VAGC = 360 mV

(1)

However, because VAGC varies as the ac AGC signal varies and also differentially around Vref, then VAGC should

have an ac signal component and a dc component. To preserve the dc and thermal tracking of the device, this

dc voltage must be generated from Vref. To apply proper bias to the AGC input, the external circuit used to

generate VAGC must combine these two voltages. Figures 6 and 7 show two circuits that will perform this

operation and are easy to implement. The circuits use a standard dual operational amplifier for AGC feedback.

By providing rectification and the required feedback gain, these circuits are also complete AGC systems.

circuit operation

Amplifier A1 amplifies and inverts the rectified and filtered AGC signal voltage VC producing output voltage V1.

Amplifier A2 is a differential amplifier that inverts V1 again and adds the scaled Vref voltage. This conditioning

makes VAGC the sum of the signal plus the scaled Vref. As the signal voltage increases, VAGC increases and

the gain of the TL026C is reduced. This maintains a constant output level.

feedback circuit equations

Following the AGC input signal (Figures 6 and 7) from the OUT output through the feedback amplifiers to the

AGC input produces the following equations:

(2)

1. AC ouput to diode D1, assuming sinusoidal signals

VO = VOP (sin (wt))

where:

VOP = peak voltage of VO

(3)

2. Diode D1 and capacitor C1 output

VC = VOP – VF

where:

VF = forward voltage drop of D1

VC = voltage across capacitor C1

(4)

3. A1 output

V1

+* R2

R1

V

C

(5)

4. A2 output (R3 = R4)

V

AGC +

R2

R1

V

C )

2

R6

R5

) R6

V

ref