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Linear Array CWDP Doppler Estimator Predictive ADC
Ultrasound BJT LNA


The presented wide-linear-range ultrasound receiver comprises a low-noise preamplifier (LNA) and a T/R switch. As shown below, the switch isolates the preamplifier from  the high-voltage pulses generated by the transmitter during the transmission mode and connects the transducer element to the preamplifier during reception. 
T/R Switch
According with the invention, the T/R switch is embedded into the LNA input structure. This provides a local feedback increasing the LNA's dynamic range. The circuit is unconditionally stable while allowing to program its input impedance.

Typically, the LNA topology consists of a differential amplifier. A bipolar differential pair  is considered to be linear as long as an input swing remains unsaturated, i.e., between the limits of ± 50 millivolts.  To expand dynamic range further, one can use a local feedback usually called “emitter degeneration”. However, selecting a larger emitter resistor increases noise.
Numerous techniques have been proposed to improve linearity (input dynamic range) of a differential amplifier.  The most effective among those methods is the one called “multi-tanh”. The idea behind the multi-tanh technique is having multiple differential pairs with different input offset bias operating in parallel. Although the multi-tanh circuits provide an extended dynamic range, they suffer from several disadvantages:

There is a need in multiple offset voltage generating circuits, a circuit for weighting output currents, and a circuit for adding the weighted output currents.
The supplementary circuits would contribute additional noise.
The value of said offset voltages is in order of 0.5 to 2 thermal potentials.  Thus, high-accuracy generation of the offset voltages becomes a technical challenge itself

To increase the linear dynamic range of a differential amplifier, we conceived using of a dedicated negative feedback  that directly controls the transconductance characteristics. As shown, flatness of the transconductance curve depends on the loop gain, G. 
The graph below illustrates two-tone IMD simulations for the linearized circuit at a ± 150 mV input swing and G = 4.
2-Tone IMD

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