Transistors and synapses: Robust, low power analog circuits in CMOS radios and the rabbit retina.

摘要

This dissertation presents transistor circuits that make up a very low power radio, and neural circuits that have been extracted from direct measurement of retinal neurons in the rabbit retina.; Running at a carrier frequency of 900MHz, the radio described here was shown to communicate up to 100 kilobits per second at ranges of 16 meters or more while consuming 1.3mW in transmit mode and 1.2mW in receive. The whole design only requires 4 external components at a cost of less than 1 dollar. Power reduction was achieved by stacking circuits to make maximum use of battery voltage, and using a single high quality inductor to resonate out capacitance on the inputs of each RF block. The receiver makes extensive use of complementary CMOS circuits for robustly high gain at low power. Simple passive switching mixers were also employed, improving the linearity of the system and permitting demodulation of 1.0 picowatt wanted signals in the face of interfering signals as large as 100muW. Part of this design incorporated a new type of current mirror that with just three additional transistors dramatically reduces the required voltage headroom required to maintain a constant current output by more than a factor of 2 with very little cost in terms of current or die area. The circuits presented here represented a new record in terms of performance at low power.; In the retina, bipolar cells are the primary analog feed-forward cells, but also participate in feedback networks thought to generate diverse signaling pathways. Strikingly, while there are at least 10 morphologically distinct classes of bipolar cell, electrophysiological measurements of these cells only showed 4 distinct types of inhibitory feedback. Also striking was the observation that the OFF cells, which receive increased excitation in response to decreases in light level, and make up fully half of the bipolar cell population, receive only one discernable type of inhibition. This inhibition increased with increased light intensity (an ON signal) and so acted to enhance the OFF response across a wide range of time scales. In contrast, ON bipolar cells (representing the other half of the bipolar cell population) received a variety of different types of inhibition. Some received OFF inhibition which acted to enhance responses in a way similar to the OFF system. Others received inhibition from within the ON system which suppressed low frequency signals but carried an apparent delay causing it to actually enhance the response to high frequency inputs. A third class (identified morphologically as rod bipolar cells) received inhibition that from within the ON system which acted exclusively to suppress responses at all frequencies. Thus, the feedback to bipolar cells is asymmetric between the ON and OFF pathways. Furthermore, these different types of inhibition were pharmacologically distinct, employing different types of inhibitory neurotransmitter.; This work elucidates the inhibitory circuitry that maintains linearity in the primary visual pathways in mammals, and further demonstrates how the retina maintains its robust functionality in the face of inevitable variability in the components of the system. Thus, as in low power radios, the primary problem to be solved in low power analog circuits in the retina is one of reliability in the face of unreliable physical components. (Abstract shortened by UMI.)