3.5 Device Bias Considerations

3.5 Device Bias Considerations

3.5.1 Device Bias Tips

  1. In circuits where a bias voltage is applied to the base of a bipolar transistor, finer control over the collector current, Ic, can be achieved by tying two large valued resistors of equal value (e.g., 10 kOhms) between the positive and negative ports on the power supplies and tapping the voltage at the center of the resistors as shown in Figure 3.2 (the voltage divider rule applies here). Since the diode current is, by definition, an exponential function of the voltage applied across its pn junction (in this case the base-emitter junction), adding a resistor has the effect of linearizing the base current which makes Ic less sensitive to small changes in the applied base voltage.

Figure 3.4 Reducing current sensitivity to base bias voltage.

  1. To monitor the drain/collector current, a multimeter set to current mode can be connected to the power supply supplying voltage to the drain or collector.
  2. When attenuators and Bias T's are used in the signal path, it is best to place the Bias T's between the attenuator and the DUT rather than place the attenuator between the DUT and Bias T. This is because the attenuators consume power and may alter the applied bias voltage to the DUT. For network analyzer measurements, an external Bias T will be required as opposed to using the one built into the network analyzer (Refer to Section 4.6 for more information on Bias T's).

3.5.2 RF Chokes & Blocking Capacitors

An RF choke inductor (RFC), or length of high impedance transmission line (preferably a quarter wavelength at the band center), is used to help isolate the RF signal from entering into the DC supply and to prevent any RF present on the DC supply from being coupled into the circuit. The reactance of the RFC should be high at the lowest frequency of operation (e.g., several hundred ohms  [1]). In addition, a large value capacitor (e.g., 1uF) connected to the DC side of the RFC to ground will provide a low impedance path to any signal that does get past the RFC. RFCs also prevent the very low AC impedance of the power supply from loading the circuit. As a rule of thumb, the total series impedance of the choke appearing between the output bias terminal and the power supply should be at least 10 times greater than that of the designed load impedance if the choke network is not to affect circuit performance (i.e., greater than 10 x nominal 50 ohm load = 500 ohms) [1].

External AC coupling capacitors should be used at the input and output of the IC to ensure that the RF loads provided to the IC do not shift the biasing. These capacitors should provide a low series impedance (typically less than 5 Ohms), throughout the frequency band over which the IC is to be used. High Q capacitors should be used at the input since any loss will add to the noise figure of the circuit. For narrowband applications, capacitors at resonance can be used, as this provides minimal insertion impedance. Typical values for blocking capacitors are on the order of 1000pF, with associated parasitic inductances of 0.5nH [1]. The value of the blocking capacitor will usually determine the lowest frequency of operation of the circuit.

The power supply should be bypassed to ground with a capacitor to keep RF off of the DC lines and to prevent gain dips or peaks in the response of an amplifier.

When multiple bypass capacitors are used, consideration should be given to potential resonances. It is important to ensure that the capacitors do not form resonant circuits when combined with additional parasitic L's and C's on the circuit board. Adding a small value resistor (a few ohms) in the bias supply line between bypass capacitors will often "de-Q" the bias circuit and eliminate the effect of resonance.

3.5.3 Supplying Power to the DUT

There are four typical methods to provide power to the DUT during probing: (1) Bias T (2) Multiple-needle probes (3) Multicontact probes (4) Single-needle positional probes. Bias T's, which are high-frequency chokes, are typically used when probing individual transistors or FETs, and allow the signal lines to be DC biased. Bias T's that have been installed in the S-parameter test set are usually used to characterize individual transistors. Needle-type probes are typically used to provide power to ICs being probed. They work well except when their inductance (5-8nH) adversely affects circuit performance as in high gain amplifiers and fast digital devices. You should model these effects in your circuit to check the result. Multicontact probes can be used in cases where the inductance associated with needle-type probes adversely affects the DUT performance.

The input and/or output bias can be coupled through a bias T for testing (such as Picosecond Pulse Labs Model 5535), or on a PCB the output bias can be supplied through a resistor. Most RF generators are AC coupled and so a coupling capacitor is not really necessary. For S-parameter measurements, there are bias T's built into most S-parameter test sets (all of the HP sets have them built in) and so this is not a problem. The 1mH number is sufficient for simulation. At RF it will look like a very large impedance compared to a 50 ohm load and is effectively out of the circuit. The typical bias T has about 8 mH of inductance and 4 ohms of series resistance. The inductor can be purchased separately, but it would be very convenient to use the bias T's that are built into the S-parameter test set of a network analyzer.

3.5.4 References

[1] Hewlett Packard, Communications Components Designer's Catalog, 1997.

[2] Cascade Microtech Application Note, "Introduction to bipolar device GHz measurement techniques"

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