Introduction

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.

 

 

The Transconductor Ideal Model

   

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-).   eq.(1)

 

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.

 

Fig. 2 OTA's basic configurations

 

The Transconductor Circuit

    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.

 

The RIAA Synthesis  

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 (1st pole);

fz1=500Hz (zero);

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

Bandwidht with inverse RIIA input network it’s shown in Fig.6. The averaged accuracy is within 0.3dB. In Fig.7 we have the simulated response without deenphasis added.

 

 

 

 

 

 

 

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

The OTA's semplicity it's well adapted for the design of high-gain phono preamps as requested by MC cartridges. In this brief description a simple two-stages split-riia MC phono transconductor with high gain illustrates the capabilities of this design concept.

 

 

What did you think of this article?
Click here to send us your comments, feedback and suggestions