Low-power, high-modulation-index amplifier for use in battery-powered device
View Patent ↗An external transmitter circuit drives an implantable neural stimulator having an implanted coil from a primary coil driven by a power amplifier. For efficient power consumption, the transmitter output circuit (which includes the primary coil driven by the power amplifier inductively coupled with the implanted coil) operates as a tuned resonant circuit. When operating as a tuned resonant circuit, it is difficult to modulate the carrier signal with data having sharp rise and fall times without using a high power modulation amplifier. Sharp rise and fall times are needed in order to ensure reliable data transmission. To overcome this difficulty, the present invention includes an output switch that selectively inserts a resistor in the transmitter output coil circuit in order to de-tune the resonant circuit only during those times when data modulation is needed. Such de-tuning allows sharp rise and fall times in the data modulation without the need for using a high power modulation amplifier. Because data modulation is typically needed for only a small percent of the time that a carrier signal is present, it is thus possible using the present invention to achieve reliable data modulation, transmission and reception without having to use a high power modulation amplifier in the transmitter.
1. A low power modulation amplifier circuit adapted to be coupled with an implantable device, comprising:
an amplifier having an output port through which a carrier signal is presented to a load, the load including an implantable coil associated with the implantable device;
a first capacitance connected between the output port of the amplifier and a voltage reference;
an antenna coil connected to the output port of the amplifier, the antenna coil comprising part of the load to which the carrier signal is presented by the amplifier;
a resistor connected to the antenna coil; and
a switch that switchably connects the resistor in circuit relationship with the antenna coil.
2. The amplifier circuit of claim 1 wherein the resistor is connected in series with the antenna coil when the switch is opened.
3. The amplifier circuit of claim 1 wherein the resistor is connected in parallel with the antenna coil when the switch is closed.
4. The amplifier circuit of claim 1 wherein the antenna coil forms part of a resonant circuit; and wherein the switch, when connected to the antenna coil, de-tunes the resonant circuit so that the resonant circuit has a Q of four or less.
5. The amplifier circuit of claim 4 wherein the carrier signal is modulated at a low duty cycle of no greater than about 4 percent.
6. The amplifier circuit of claim 5 wherein the carrier signal is modulated at a low duty cycle of no greater than about 2 percent.
7. The amplifier circuit of claim 1 wherein the switch comprises a plurality of switches connected in parallel.