Patent Application
20080007343
The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:
With reference to
There are other ways of accomplishing the controlled current source 31, but in accordance with the illustrated embodiment, you need a tunable current source 33 and a mirror 32, which present the current Icurrentsource to the front end circuit 11. The fourth transistor 37 Pdiode provides the control voltage that tells the current mirror transistor, i.e. the third transistor 36, Pmirror what current to deliver, which is fundamental current source construction. In use the fourth transistor 37 is the diode, i.e. the side that the reference current (Icurrentsource) enters, and the third transistor 36 is the mirror. If the third and fourth transistors 36 and 37 are identical, then the voltage across the diode, i.e. the fourth transistor 37, will generate the exact voltage necessary to control the mirror, i.e. the third transistor 36, to put out the same amount of current.
The present invention enables a designer to decouple the design aspects of the Rc collector resistor 24 into two components, i.e. separate the setting of the open loop gain A, from the desired amount of current IFT running through the first transistor 23. The 2 mA desired bias current IFT through the first and second transistors 23 and 28 can be set by the current source 33, and the open loop gain can be set by choice of the collector resistance Rc 24, in this case, 200 Ohms.
The embodiment described above is intended to cancel out unwanted power supply variation; however, a designer might also want to mitigate other second order circuit effects, including, but not limited to, temperature, received signal strength, and data rate, by tuning the characteristics of the current source 33. The emitter resistance of the first transistor 25 is temperature dependent, but the collector Rc resistor 24 may have no temperature coefficient, or the temperature coefficient of the collector resistor 24 may be in the opposite direction to the temperature coefficient of the transistor emitter resistance. Under these conditions, the open-loop gain of the front end circuit 11 will vary greatly from cold to hot temperatures. In these cases, the designer can construct Icurrentsource out of a mixture of currents that are flat with temperature and currents that are proportional to absolute temperature (PTAT) to arrive at a combination that will provide consistent open-loop gain over the designed temperature range. It is well known in the industry that currents of arbitrary temperature coefficient, positive or negative can be generated by adding and subtracting the right proportions of flat and PTAT currents. Therefore, a designer can tune the temperature performance of the front end circuit 11 to achieve any desired performance. A PTAT current may be appropriate for canceling out the decreasing emitter resistance with temperature. A different temperature profile might be desirable for canceling out the temperature coefficient of the collector Rc resistors 24 and 30. This can be done internal to a control chip provided with the TIA PCB at the time of fabrication as temperature changes in circuit components are generally well modeled. A designers simulations will show the temperature dependence of the open-loop gain, for instance, which can then be mitigated with the appropriate recipe of flat, PTAT and NTAT currents built on-chip. A temperature monitor can also be provided for sending temperature measurements to an external control processor, which tunes the current source 33 accordingly. The latter method is possible, but more expensive to implement and generally not necessary.
Alternatively, if the open loop gain A is too high at cold temperatures, and too low at hot temperatures, the current (I) from the current source 33 can be made proportional to absolute temperature (PTAT) so that more current could be supplied at hot and less at the cold condition, to ensure the open loop gain A is maintained substantially constant, e.g. ±5%. As above, a temperature monitor can be provided for sending temperature measurements to a control processor, which tunes the current source 33 accordingly. Alternatively, a temperature profile can be predetermined and saved in the control processor to tuning the current source 33.
In an alternate embodiment, shown in
In another alternate embodiment similar to the one discussed above with reference to
For a power supply variation of 600 mV, collector resistor 24 of 200 Ohms and feedback resistor 25 of 400 Ohms, the current required would be about 1 mA, ⅓ of the previous embodiment. This implementation suffers one key shortcoming for high-speed applications: the pd_anode node is highly sensitive to capacitance. Each circuit element connected to pd_anode adds capacitance and worsens the performance.
