Analog Devices manual AD600/AD602

AD600/AD602
		+5V
		R3
		46.4kΩ
		R4						+5V
		3.74kΩ
							VG'	AD590	300∝A		+5V
									(at 300K)
RF	C1LO	1	16	C1HI							FB
INPUT
	A1HI			A1CM			C2				0.1∝F
		2	15		C4	R1				+5V DEC
							1∝F
	A1LO	A1		A1OP	0.1∝F
						100Ω			Q1
		3	14
									2N3904
	GAT1			VPOS	+5V					–5V DEC
		4	13			C1
					DEC						0.1∝F
		REF				100pF		R2
	GAT2			VNEG	–5V		C3		VPTAT		FB
		5	12				15pF	806Ω
					DEC
	A2LO			A2OP				1%		RF
		6	11								–5V
										OUTPUT
	A2HI	A2		A2CM							POWER SUPPLY
		7	10
											DECOUPLING NETWORK
	C2LO			C2HI
		8	9
		AD600

Figure 15. This Accurate HF AGC Amplifier Uses Just Three Active Components

A simple half-wave detector is used, based on Q1 and R2. The average current into capacitor C2 is just the difference between the current provided by the AD590 (300 μA at 300 K, 27°C) and the collector current of Q1. In turn, the control voltage VG is the time integral of this error current. When VG (and thus the gain) is stable, the rectified current in Q1 must, on average, ex- actly balance the current in the AD590. If the output of A2 is too small to do this, VG will ramp up, causing the gain to in- crease, until Q1 conducts sufficiently. The operation of this control system will now be described in detail.

First, consider the particular case where R2 is zero and the out- put voltage VOUT is a square wave at, say, 100 kHz, that is, well above the corner frequency of the control loop. During the time VOUT is negative, Q1 conducts; when VOUT is positive, it is cut off. Since the average collector current is forced to be 300 μA, and the square wave has a 50% duty-cycle, the current when con- ducting must be 600 μA. With R2 omitted, the peak value of VOUT would be just the VBE of Q1 at 600 μA (typically about

700mV) or 2 VBE peak-to-peak. This voltage, hence the ampli-

tude at which the output stabilizes, has a strong negative tem- perature coefficient (TC), typically –1.7 mV/°C. While this may not be troublesome in some applications, the correct value of R2 will render the output stable with temperature.

To understand this, first note that the current in the AD590 is closely proportional to absolute temperature (PTAT). (In fact, this IC is intended for use as a thermometer.) For the moment, continue to assume that the signal is a square wave. When Q1 is

conducting, VOUT is the now the sum of VBE and a voltage which is PTAT and which can be chosen to have an equal but opposite

TC to that of the base-to-emitter voltage. This is actually noth- ing more than the “bandgap voltage reference” principle in thinly veiled disguise! When we choose R2 such that the sum of the voltage across it and the VBE of Q1 is close to the bandgap voltage of about 1.2 V, VOUT will be stable over a wide range of temperatures, provided, of course, that Q1 and the AD590 share the same thermal environment.

Since the average emitter current is 600 μA during each half- cycle of the square wave, a resistor of 833 Ω would add a PTAT voltage of 500 mV at 300 K, increasing by 1.66 mV/°C. In prac- tice, the optimum value of R2 will depend on the transistor used, and, to a lesser extent, on the waveform for which the tem- perature stability is to be optimized; for the devices shown and sine wave signals, the recommended value is 806 Ω. This resistor also serves to lower the peak current in Q1 and the 200 Hz LP filter it forms with C2 helps to minimize distortion due to ripple in VG. Note that the output amplitude under sine wave condi- tions will be higher than for a square wave, since the average value of the current for an ideal rectifer would be 0.637 times as large, causing the output amplitude to be 1.88 (= 1.2/0.637) V, or 1.33 V rms. In practice, the somewhat nonideal rectifier results in the sine wave output being regulated to about 1.275 V rms.

An offset of +375 mV is applied to the inverting gain-control inputs C1LO and C2LO. Thus the nominal –625 mV to

+625 mV range for VG is translated upwards (at VG´) to –0.25 V for minimum gain to +1 V for maximum gain. This prevents Q1 from going into heavy saturation at low gains and leaves suffi- cient “headroom” of 4 V for the AD590 to operate correctly at high gains when using a +5 V supply.

In fact, the 6 dB interstage attenuator means that the overall gain of this AGC system actually runs from –6 dB to +74 dB. Thus, an input of 2 V rms would be required to produce a 1 V rms output at the minimum gain, which exceeds the 1 V rms maximum input specification of the AD600. The available gain range is therefore 0 dB to 74 dB (or, X1 to X5000). Since the gain scaling is 15.625 mV/dB (because of the cascaded stages) the minimum value of VG´ is actually increased by 6 × 15.625 mV, or about 94 mV, to –156 mV, so the risk of saturation in Q1 is reduced.

–10– REV. A