1172 
MONITORING 
V 
POWER 
SUPPLY 
RF 
SWITCH 
6 MHz 
OSCILLATOR 
-/ V 
TRANSDUCERS 
6 MHz AM 
RECEIVER 
INTERNAL 
EXTERNAL 
100 MHz FM 
XMITTER 
RECORDER 
DEMODULATOR 
100 MHz FM 
RECEIVER 
FLOWMETER SYSTEM 
Figure 3. — Block diagram of implantable CW Doppler flowmeter. 
Low Noise AM Receiver Circuit 
A schematic diagram of the low noise AM 
receiver circuit is illustrated in Figure 6; the 
circuit consists of an RF amplifier stage (Qi 
and Q2) a phase splitter (Q7 and Qg) and a bal- 
anced mixer (Q3, Q4, Q5, Qg, Qo, Qio and Qn). 
Several key features of the design are: close 
matching of monolithic transistors Qi and Qo 
and resistors R2 and R3 is exploited in estab- 
lishing the collector current of Qo ; capacitor 
C2 is a small area monolithic element consisting 
of emitter junction, collector junction and emit- 
ter oxide capacitance in parallel; nevertheless, 
the resonant frequency of L, and C2 is inde- 
pendent of supply voltage since Lj maintains a 
dc voltage of 0 v across Co; Qg and Q5 are op- 
erated at 0 V collector-base voltage and Q4 and 
Qe are actually operated with a very small for- 
ward bias on the collection junction in order 
to conserve supply voltage. Also, in order to 
provide equal local oscillator amplitude to both 
halves (Qn and Qio) of the balanced mixer from 
two points at diiferent dc potentials in the phase 
splitter (Qs), the monolithic capacitors C4 and 
C5 consist of emitter oxide capacitance only. 
Finally, the entire concept of using a balanced 
mixer is utterly unattractive unless low-cost 
closely-matched monolithic elements are avail- 
able. The key advantage of a balanced mixer is 
that the high level carrier is essentially can- 
celled by the balanced circuit and does not ap- 
pear at the audio output. Consequently, filter 
circuitry to remove the carrier becomes unneces- 
sary; the only discrete components needed in 
the receiver are Ci and Li. 
This low noise receiver operates at room 
temperature over a supply voltage range of 1.0 
to 1.35 V. Its current drain is 1.0 ma; its sen- 
sitivity is 1.0 /iv and the conversion gain is 
