Analog Devices AD600 Low Noise, 6 dB Preamplifier, Table I. Measured Preamplifier Performance

AD600/AD602

	C1LO
	A1HI
	A1LO
	GAT1
VIN	GAT2

GAIN-CONTROL

VOLTAGE

VG

1 16

2				15
		A1
3		14
4			13
		REF
5	12

C1HI

A1CM

A1OP100Ω

VPOS

+5V

VNEG

–5V

VOUT

An inexpensive circuit, using complementary transistor types chosen for their low rbb, is shown in Figure 14. The gain is de- termined by the ratio of the net collector load resistance to the net emitter resistance, that is, it is an open-loop amplifier. The gain will be X2 (6 dB) only into a 100 Ω load, assumed to be provided by the input resistance of the X-AMP; R2 and R7 are in shunt with this load, and their value is important in defining the gain. For small-signal inputs, both transistors contribute an equal transconductance, which is rendered less sensitive to sig-

A2LO

6		11
	A2

A2OP

100Ω

50Ω

nal level by the emitter resistors R4 and R5, which also play a dominant role in setting the gain.

A2HI

C2LO

7 10

8 9 AD600 or AD602

A2CM

C2HI

This is a Class AB amplifier. As VIN increases in a positive di- rection, Q1 conducts more heavily and its re becomes lower while that of Q2 increases. Conversely, more negative values of

Figure 13. An Ultralow Noise VCA Using the AD600 or AD602

A Low Noise, 6 dB Preamplifier

In some ultrasound applications, the user may wish to use a high input impedance preamplifier to avoid the signal attenua- tion that would result from loading the transducer by the 100 Ω input resistance of the X-AMP. High gain cannot be tolerated, because the peak transducer signal is typically ± 0.5 V, while the peak input capability of the AD600 or AD602 is only slightly more than ± 1 V. A gain of two is a suitable choice. It can be shown that if the preamplifier’s overall referred-to-input (RTI) noise is to be the same as that due to the X-AMP alone (1.4 nV/ √Hz), then the input noise of a X2 preamplifier must be √(3/4) times as large, that is, 1.2 nV/√Hz.

	+5V
	R1	1∝F
	49.9Ω
	R2
	174Ω
		Q1
		MRF904
1∝F		R3
		562Ω
			0.1∝F
R4
	–5V
42.2Ω
VIN			INPUT
R5	+5V
			GROUND	100Ω
42.2Ω
			0.1∝F	RIN OF X AMP
		R6
1∝F
		562Ω
		Q2		OUTPUT
				GROUND
		MM4049

R7

Ω 1∝F 174

R8

49.9Ω

–5V

Figure 14. A Low Noise Preamplifier for the AD600 and AD602

VIN result in the re Of Q2 decreasing, while that of Q1 increases. The design is chosen such that the net emitter resistance is es- sentially independent of the instantaneous value of VIN, result- ing in moderately low distortion. Low values of resistance and moderately high bias currents are important in achieving the low noise, wide bandwidth, and low distortion of this preamplifier. Heavy decoupling prevents noise on the power supply lines from being conveyed to the input of the X-AMP.

Table I. Measured Preamplifier Performance

Measurement		Value	Unit
Gain (f = 30 MHz)		6	dB
Bandwidth (–3 dB)		250	MHz
Input Signal for
1 dB Compression		1	V p-p
Distortion
VIN = 200 mV p-p	HD2	0.27	%
	HD3	0.14	%
VIN = 500 mV p-p	HD2	0.44	%
	HD3	0.58	%
System Input Noise		1.03	nV/√	Hz
Spectral Density (NSD)
(Preamp plus X-AMP)			kΩ
Input Resistance		1.4
Input Capacitance		15	pF
Input Bias Current		± 150	μA
Power Supply Voltage		± 5	V
Quiescent Current		15	mA

A Low Noise AGC Amplifier with 80 dB Gain Range

Figure 15 provides an example of the ease with which the AD600 can be connected as an AGC amplifier. A1 and A2 are cascaded, with 6 dB of attenuation introduced by the 100 Ω resistor R1, while a time constant of 5 ns is formed by C1 and the 50 Ω of net resistance at the input of A2. This has the dual effect of (a) lowering the overall gain range from {0 dB to 80 dB} to {6 dB to 74 dB} and (b) introducing a single-pole low-pass filter with a –3 dB frequency of about 32 MHz. This ensures stability at the maximum gain for a slight reduction in the over- all bandwidth. The capacitor C4 blocks the small dc offset volt- age at the output of A1 (which might otherwise saturate A2 at its maximum gain) and introduces a high pass corner at about 8 kHz, useful in eliminating low frequency noise and spurious signals which may be present at the input.

REV. A –9–