OTA (operational transconductance amp.) [grise, lecture14, - PowerPoint PPT Presentation

OTA (operational transconductance amp.) [grise, lecture14, AN6077.1, LM13700, (AN6668.2)]

 The OTA is a transconductance type device  the input voltage controls an output current by means of the device transconductance g m  g m is controlled by an external current, the amplifier bias current, I abc  an output voltage can be derived from the output current by simply driving a resistive load for a BJT OTA: g m =I abc /2V T which is the transconductance of a BJT differential pair

 Basic voltage amp (with feedback; no load here!)... 𝑆 𝑗𝑜 = 𝑕 𝑛 𝑆 1 + 1 𝑕 𝑛  and if g m R 1 >>1, g m R 2 >>1 𝑆 𝑗𝑜 ≅ 𝑆 1

An all-OTA transistor (note: the first OTA is without feedback!)

With OTAs it is possible to design active filters which • can be controlled (via the I abc input) over a number of key parameters e.g. (the second OTA is configured as a voltage variable • resistor) 1 1 2 2 2 another example: • V

yet another example: •

 Internal structure (simplified) W, X, Y, and Z are current mirrors

 Internal structure (LM13700) • D 1 , D 4 , D 5 , and D 6 are diode- connected BJT • D 2 , D 3 : see later • current mirrors are Wilson • Q 12 and Q 13 are an optional output buffer

(Old) limitation: input voltage swing as for any BJT differential pair, the input voltage swing • is limited to about 25 mV ( the problem applies if the OTA is being used in open-loop configuration and g m is small ) Transfer characteristics ( I out vs V in ):

 Improvement for the input voltage swing [LM13700] recent OTAs use internal linearizing diodes at the input • Transfer characteristics:

suppose the input signal is a current I S (i.e. R gen is large) • neglect base currents • = D V on the baes V in = D V on the diodes = D V on the bases => V in =

Non-idealities (besides those of conventional op-amp) change in output offset with I ABC • change in output offset with I diodes • change in gain with I diodes • ... •