Transconductance amplifier

A transconductance amplifier (VC-OPV or VC-OP), also known as an operational transconductance amplifier (OTA), is a special operational amplifier that converts the differential voltage at the two inputs into a proportional output current. Like other operational amplifiers, it is usually implemented as an integrated circuit .

General

Circuit symbol of a transconductance amplifier

Circuit symbol of an OTA, as it is common in the English-language specialist literature

In contrast to the classic operational amplifier with a low-resistance voltage output, the transconductance amplifier has a high-resistance current output . If necessary, an impedance converter can be connected to the outputs in order to obtain a voltage output .

Further differences to the operational amplifier with voltage output are:

1. VC-OPs are only implemented internally with transistors and diodes without ohmic resistors . Like all other OP types, they can be built with bipolar transistors or field effect transistors .
2. A VC-OP can be operated linearly with an open loop without external negative feedback . This is possible because the resistance that is absolutely necessary at the output influences the output voltage and saturation of the VC-OP can be avoided by selecting an appropriate external resistor.

A special feature resulting from this is that analog circuits such as analog filters can be set up with an OTA without ohmic resistors. Ohmic resistances with corresponding accuracy are difficult to implement in integrated circuits from a manufacturing point of view. OTAs therefore play a role as a basic element in freely programmable analog circuits such as the Field Programmable Analog Array (FPAA).

An OTA can be used as an analog multiplier if the accuracy requirements in the LF range are not too high (see function). This z. B. modulators , voltage-controlled volume controls, compander, etc. can be implemented relatively easily.

The first transconductance amplifier CA3080 was manufactured in 1969 by the company RCA . The CA 3080 has the disadvantage that the distortion increases sharply at input voltages above 25 mV. Today, OTAs are available from various manufacturers. Examples are the LM13700 from National Semiconductor or the LT1228 from Linear Technology . As an independent circuit, however, the OTA is not as important as the operational amplifier with voltage output.

function

The basic function is given by:

${\displaystyle I_{\mathrm {a} }=(U_{\mathrm {+} }-U_{\mathrm {-} })\,g_{\mathrm {m} }=U_{\mathrm {D} }\,g_{\mathrm {m} }}$

with the output current , the input differential voltage and the transfer rate or transconductance . The transmission slope has the dimension of an electrical conductance . ${\displaystyle I_{\mathrm {a} }}$ ${\displaystyle U_{\mathrm {D} }}$ ${\displaystyle g_{\mathrm {m} }}$

An output voltage can be obtained through an external load resistor : ${\displaystyle U_{\mathrm {a} }}$ ${\displaystyle R_{\mathrm {a} }}$

${\displaystyle U_{\mathrm {a} }=I_{\mathrm {a} }\,R_{\mathrm {a} }}$

The voltage gain is given as: ${\displaystyle G}$

${\displaystyle G={\frac {U_{\mathrm {a} }}{U_{\mathrm {D} }}}=R_{\mathrm {a} }\,g_{\mathrm {m} }}$

The conductance of the amplifier can typically be adjusted over three to four decades. A separate control input (usually not shown in the circuit symbols) is provided for this purpose, at which the operating point can be adjusted with a control current . The index commonly used in data sheets and specialist literature stands for Amplifier Bias Current . This current has the following relationship with the rate of transmission: ${\displaystyle g_{\mathrm {m} }}$ ${\displaystyle I_{\mathrm {ABC} }}$ ${\displaystyle \mathrm {ABC} }$

${\displaystyle g_{\mathrm {m} }={\frac {I_{\mathrm {ABC} }}{2\,U_{\mathrm {T} }}}}$

The transfer function can also be written like this:

${\displaystyle I_{\mathrm {a} }=U_{\mathrm {D} }\cdot g_{\mathrm {m} }=U_{\mathrm {D} }\cdot {\frac {I_{\mathrm {ABC} }}{2\,U_{\mathrm {T} }}}}$

The output current is therefore proportional to the product of the differential voltage at the input times the bias current within the limits described.

The temperature voltage that occurs has a value of approximately 25 mV at room temperature. ${\displaystyle U_{\mathrm {T} }}$

literature

• Lutz v. Wangenheim: Active Filters and Oscillators . 1st edition. Springer, 2008, ISBN 978-3-540-71737-9 .

Individual evidence

1. ↑ Eugene M. Zumchak: A Short Discussion of the Operational Transconductance Amplifier (OTA) , 1999, [1]
2. ↑ Data sheet CA3080 (English; PDF; 973 kB)
3. ↑ Laborblätter, Elrad 1984, Heise Verlag issue 1, p. 52
4. ↑ data sheet LM13700. Retrieved January 15, 2019 . 
5. ↑ data sheet LT1228. Retrieved January 15, 2019 .