Colpitt’s Oscillator frequency estimation calculator

The Colpitt’s oscillator is a useful building block that is simple to build using a very few components. However, it is somewhat difficult to design and simulate. Signal Processing Group has recently released a simple Javascript calculator that provides an estimation of the frequency of the design. However, after this calculator is used the user must still “tweak” the design on the board. Please access the calculator at

Hybrid pi model of the bipolar transistor

The hybrid pi model of the bipolar transistor is a popular model used for small signal modeling of the bipolar. A paper released by Signal Processing Group describes the components that constitute the model and presents first order expressions for their calculation. The paper is available for free on the Signal Processing Group website and can be accessed from the “complementary” menu. Please visit. In addition a javascript calculator is planned for release from Signal Processing Group that can be used by a user to make the calculations easier.

The Hybrid Pi model of Bipolar ( and MOSFET) devices

The hybrid Pi model is a popular model used to analyze small signal performance of active circuits based on bipolar and MOSFET transistor and other devices with appropriate modifications. The following are the descriptions of its component parts:

  1. It has three terminals ( the schematic shows a bipolar model with base, emitter and collector as its terminals. C = collector, B = base and E= emitter.
  • rbb is the base spreading resistance. The resistance between the base contact and the internal base of the transistor.
  • rb’e is the base to emitter resistance.  Represents the base current required to make up for recombination of minority carriers in the base region.
  • Ce is the emitter base diffusion capacitance.
  • Cc and rb’c represents the Early effect which accounts for the finite collector to emitter output resistance.
  • gce/ro represents the output impedance ( conductance) of the device.

This complete model can be simplified as needed for active circuits using these devices.

Colpitt’s oscillator output voltage calculation using ” describing functions”

Colpitt’s oscillator is a popular oscillator circuit which is constructed from an inductor, two capacitors, some resistors, and an active device such as a bipolar or a MOSFET, and generally some trial and error !!! Of particular difficulty is the calculation a priori, of the output voltage of the oscillator. However, a technique using the describing function of a bipolar ( or MOSFET or JFET, IGFET etc) makes it possible to calculate the output voltage to a close estimate. ( Perhaps some optimizing needs to be done). For the uninitiated here is a simple definition of a describing function. Assume you have a non linear device which you would need to analyze, using linear tools. How would you do it? In the time domain it is definitely non linear but if you switch to the frequency domain linear analysis tools can be applied. How? Hit the non linear system with a sinusoid of a particular amplitude and frequency. Since the system is non linear it will generate, at its output, a number of sinusoids. Lets pick the output sinusoid that matches our input frequency and all its associated characteristics ( phase etc). This becomes a describing function. Ignore the rest of the outputs. Then linear analysis tools can be applied to this describing function and results obtained. Please visit our website for more technical info and other information.

Signal Processing Group status for October 2021

Signal Processing Group is alive and well and operating as usual. Contact us for analog, RF, Microwave product design, ASICs as well as modules. We can now do 3D modeling using Shapr3d and Solidworks, so we can produce enclosures. Latest work on RFMW is a K band amplifier. Other RFPA’s are a S band amplifier, Find us on Digikey as well where we have started to sell our products starting with a wideband 1dB NF LNA and a wideband RF detector. Many more products to follow. Take a look at our blog for valuable tech. info as well. or just go to our website at any time 24/7. We look forward to your visit. You can contact us via email at anytime.

HFSS as a tool to extend EM analysis of RFMW circuits and systems.

Having designed RFMW circuits, ASICs, RFICs and modules for a long time and used most simulators to do the analysis ( the latest in line being ADS and Microwave Office) we added the ANSYS tool HFSS to the repertoire. Although it does not seem to be used as widely as the SPICE based simulators we found it to be of great value in our analysis and a great help in deriving parameters not produced by either of the two aforementioned CAD tools. It provides EM analysis, power analysis, fields and radiation analysis which is a real help in our RFMW design efforts. The philosophy of the tool is slightly different from the usual tools. However, if you want to reduce the risk on your design to a irreducible minimum then HFSS and some of the other ANSYS tools can come in real handy. More on the HFSS tool analysis in this blog to follow. Please visit our website for more analog, RFMW ASIC and module information.

Calculate the gain of a MOSFET in saturation in the strong inversion region given the drain current and the effective gate voltage

The product of the MOSFET transconductance ( KP/KN) and the aspect ratio ( W/L) is being labeled as the gain of the operating MOSFET. A simple calculator to do this is now available for a first order estimate on the Signal Processing Group website. Please check the “complementary” item menu in the SPG website for this and other items of interest.

Calculate the current in the saturation region of a MOSFET in strong inversion

In analog CMOS design the MOSFET is usually operated in saturation where the IDS – VDS curves are close to being flat. The slight slope leads to the definition of the channel length modulation parameter. A simple calculator has been released by Signal Processing Group to allow a first order calculation of this current as starting points of a design. Please visit the SPG website and look under the “calculators” menu item.