IR circuits: increasing sensitivity of simple analog receiver IR LED
Most infra-red chips used for appliance remote-control accept a very wide range of optical power levels. They do so by including automatic gain control (AGC) circuits.

 
The AGC block is a linear amplifier whose gain is variable: gain is adjusted by a variable DC voltage or DC current from a control circuit. A variable gain circuit might consist of an analog multiplier. One input is the 38 kHz signal from the optical detector, the other input is the DC control signal.
 For large amplitude signals, the control circuit adjusts this DC voltage or DC current to compress signal amplitude by reducing amplifier gain.

For some IR receiver chips, the control signal might also be used by the demodulator, from which the digital output is derived.



A two-pin phototransistor (with 3k load resistor) may be made more sensitive by adding a linear amplifier. Since a 38 kHz signal is of interest, the amplifier input would be AC coupled through a capacitor, so that daylight (a near-DC signal) would not be amplified. This function is more precisely accomplished by the chip's Bandpass filter block.
A conceptual linear amplifier circuit (below). Note that this is an extremely simple amplifier with no gain control:







simulate this circuit &ndash; Schematic created using CircuitLab



Since the TSOP1738 is a monolithic chip, none of its internal blocks are accessible (no external pins other than Vs, OUT, and GND are provided). It is a single-purpose chip.