Featured Post

High CMRR Instrumentation Amplifier (Schematic and Layout) design for biomedical applications

Instrumentation amplifiers are intended to be used whenever acquisition of a useful signal is difficult. IA’s must have extremely high input impedances because source impedances may be high and/or unbalanced. bias and offset currents are low and relatively stable so that the source impedance need not be constant. Balanced differential inputs are provided so that the signal source may be referenced to any reasonable level independent of the IA output load reference. Common mode rejection, a measure of input balance, is very high so that noise pickup and ground drops, characteristic of remote sensor applications, are minimized.Care is taken to provide high, well characterized stability of critical parameters under varying conditions, such as changing temperatures and supply voltages. Finally, all components that are critical to the performance of the IA are internal to the device. The precision of an IA is provided at the expense of flexibility. By committing to the one specific task of

Basic low power AM transmitter

This transmitter is basic but allows transmission of audio to an AM radio. It consists of an RF oscillator operating in the AM broadcast band, together with a modulator stage, which mixes the incoming audio and the RF. A signal appears on the output, which has an AM component that can be picked up on a nearby AM radio receiver.



The transmitter consists of oscillator stage Q1 and modulator/buffer stage Q2. Q1 is biased via R1, R2, and R3. L1, C3, and C4 form the tank circuit with feedback network C3-C4 providing feedback to the emitter of Q1. RF voltage at the junction of C3 and L1 drives buffer/modulator stage Q2. Q2 is biased by base current produced by RF rectification in the base emitter junction of Q2. C6 is an RF and AF bypass capacitor. C9, C10, and L2 form the tank circuit for the collector of Q2. RF is taken from the junction of C9 and C10 and fed to a short-wire antenna. Audio is fed to modulator Q2 via C8 and isolation resistor R5 and mixes with the RF signal in the collector circuit of Q2, producing a signal that has sum and difference frequencies if the RF carrier and AF input  along with the carrier signal.
An AM signal appears at the collector of Q2. Audio with an RMS voltage equal to about 0.7 times the collector voltage of Q2 is needed for full modulation of the output. Because of the high level of audio needed, the modulation obtained from this circuit is somewhat limited with conventional audio sources because several volts of audio into a few hundred ohms is needed. The circuit demonstrates the principle of an AM transmitter, however, and with a suitable audio drive level, produces a well modulated AM signal.

Comments