METHOD OF INCREASING POWER SUPPLY REJECTION RATIO AT HIGH FREQUENCIES USING A NOVEL STATIC LOW PASS FILTER
Publication Date: 2003-May-06
The IP.com Prior Art Database
Keywords PSSR, low-pass filter, PMOS, transistor, power supply
PSSR, low-pass filter, PMOS, transistor, power supply
Power supplies are never perfect, thus the continuous signal cannot be considered as totally static. Small dynamic changes or very slow variations can and do occur resulting in variations in output voltage or current. These variations increase power consumption, degrade gain performance and increase statistical Gaussian law width when multi-cross cases (fabrication process quality, temperature, supply voltages) are involved. The above variations are detrimental to fixed output voltage generation in cell phone, audio amplifiers and radio-frequency [RF] module applications.
Furthermore, in order to increase power supply rejection ratio (PSSR) it is necessary to obtain output voltage independence from the above mentioned static and dynamic supply changes. To this end, additional complex and costly circuitry is typically required to increase the PSSR. Unfortunately, such additional circuitry typically affects other parameters defined in customer electrical specifications. For example, when slow power supply changes occur, a high pass filter is typically employed. For fast power supply changes such as alternative coupling (AC) pollution, a low pass filter is typically required.
Another typical solution includes addition of PMOS current sources in series with the electrical circuit under protection. While efficient (the polarization current is fairly independent from supply voltage) these PMOS current sources behave as high pass filters allowing detrimental AC pollution to be injected into other circuitry through grid-source and grid-drain capacitors. To counter this, such circuitry is often connected to a general chip enable or shutdown signal to allow operation as required saving energy.
In another solution, multiple transistors are used as switches to ensure every current branch can be opened. However, this solution requires NMOS transistors, which when the circuit is enabled allows detrimental AC pollution to be injected into internal circuitry.
Thus for at least the above reasons, a new low cost, simple method of providing high power supply rejection ratio at high frequencies is desirable.
Figure 1 shows an example of a new amplifier with shutdown [Shutdown, Shutdown_barre, Shdn_bar_Nmos] using NMOS transistor. Shdn_bar_Nmos signal (A) is connected to an NMOS transistor which allows an input signal to be sent to amplifier. This signal must be “high level” to allow operation. Shdn_bar_Nmos is also connected to another NMOS transistor (B) , which sets polarization current for amplifier. This signal must be “high level” when operation is required. Nevertheless, this high level can be hazardous for internal circuitry if connected to power supply due to AC pollution inje...