A
transconductor also called Operational Tranconductance Amplifier (OTA) is a voltage to current converter such
that
iout=Gm*vin
where Gm is the transconductance gain.
OTAs are emerging as attractive alternative to OP-Amp based design for a
broad range of motivations:
1)
OTAs are often single stage circuits;
2)
High frequencies behaviour (mainly related to the phase linearity) is
close to the ideal.
3)
Complex transfer function can be obtained easily.
In this article I explore the possibility offered by a hybrid transconductor stage used as basic block for the design of a high gain MC Phono Preamp. Other features include the use of vacuum triodes and a low voltage operation mode.
Fig. 1 OTA symbol and its equivalent circuit
The
symbol for the OTA is shown in Fig. 1. Basically the OTA is seen as a
differential voltage controlled current source:
iout
=Gm*(v+-v-).
In Fig. 2 is shown the great versatility of OTA in terms of obtainable
building blocks. However for our purposes is the arrangement in (c) to be used.
This arrangement realizes a non inverting integrator.
In fig.3 is shown the circuit of the used transconductor. This circuit uses a large cathode resistor in order to linearize the Gm of the differential pair for a given modulation index (iout/Ibias) and basically is a two gain stages device.
Fig. 3 OTA's circuit used
The common emitter output stage raises the gain for the required application and translate the dc voltages in order to have the a minimum dc offset. In some full solid state application the resistor can be substitued by a mosfet operating in a linear (triode) region. This last arrangement permits to control the gain and poles distribution.
Fig. 4 OTA's units for Split RIIA Synthesys
As illustrated in an other section of this site the Transfer
Function in the s-domain of the RIAA deemphasis is:
|
eq. (2) |
where:
|
eqs. (3) |
are the
complex variables and time costants involved in the process. In the
frequency domain the time costants t1,
t2 and
t3 are associated at the following
frequencies:
fp1=50Hz
fz1=500Hz
fp2=2123Hz
(2nd pole).
The pair
(fp1,fz1) can be synthesized by OTA arrangment in Fig 4a
and fp2 with the basic integrator arrangement newly depicted in
Fig.4b.
The Split RIIA MC Phono Preamp Stage.
The complete schematic of the simulated MC Phono Preamp is shown in Fig 5.
Fig. 5 MC Phono - Simulated Schematic
A
supertriode stage in ultrapath-mode is added for impedance matching
purposes. The magnetic unit is a SOT2 transformer in
m-metal
core. The 1st stage it’s used to
synthesize fp1 and fz1,
the second one synthesizes
fp2.
Frequency Performances
Fig.6 Deenphasised RIIA Frequency Response
The gain at 1kHz is 73 dB.
Fig.7 RIIA Frequency Response
Time-Domain
Performances
A parametric simulation on input voltage level variations shows the behaviour in
terms of THD depicted in Fig.7. You can observe a relatively high value in the
THD for input voltage higher than 50mV. However lower value can be
easily obtained
operating a different biasing for the ultrapath stage. This manipulation however
sacrifices a bit of bandwidth and accuracy.
Fig.8 THD vs input voltage
Conclusions
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