AD600/AD602
| C1LO |
| A1HI |
| A1LO |
| GAT1 |
VIN | GAT2 |
VOLTAGE
VG
1 16
2 |
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| 15 | |
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| A1 |
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3 |
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| 14 | ||
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4 |
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| 13 | ||
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| REF |
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5 | 12 |
C1HI
A1CM
A1OP100Ω
VPOS
+5V
VNEG
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
A2LO |
6 |
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| 11 |
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| A2 |
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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
| +5V |
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| R1 | 1∝F |
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| 49.9Ω |
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| R2 |
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| 174Ω |
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| Q1 |
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| MRF904 |
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1∝F |
| R3 |
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| 562Ω |
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| 0.1∝F |
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R4 |
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42.2Ω |
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VIN |
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| INPUT |
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R5 | +5V |
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GROUND | 100Ω | ||||
42.2Ω |
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| 0.1∝F | RIN OF X AMP | ||
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| R6 | |||
1∝F |
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| 562Ω |
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| Q2 |
| OUTPUT | |
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| GROUND | ||
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| MM4049 |
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R7
Ω 1∝F 174
R8
49.9Ω
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
Table I. Measured Preamplifier Performance
Measurement |
| Value | Unit | ||
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Gain (f = 30 MHz) |
| 6 | dB | ||
Bandwidth |
| 250 | MHz | ||
Input Signal for |
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1 dB Compression |
| 1 | V | ||
Distortion |
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VIN = 200 mV | HD2 | 0.27 | % |
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| HD3 | 0.14 | % |
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VIN = 500 mV | HD2 | 0.44 | % |
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| HD3 | 0.58 | % |
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System Input Noise |
| 1.03 | nV/√ | Hz |
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Spectral Density (NSD) |
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(Preamp plus |
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| kΩ | ||
Input Resistance |
| 1.4 | |||
Input Capacitance |
| 15 | pF | ||
Input Bias Current |
| ± 150 | μA | ||
Power Supply Voltage |
| ± 5 | V | ||
Quiescent Current |
| 15 | mA | ||
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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
REV. A |