## Biasing ERA Amplifiers
The Mini-Circuits ERA series of amplifiers are monolithic devices using gallium arsenide HBT (heterojunction bipolar transistor) technology. The internal circuit configuration is a Darlington pair, embedded in a resistor network. The resistors determine the DC operating point of the transistors, and provide feedback to set RF gain, bandwidth, and input and output impedances to optimum values. The ERA is a 2-port device: RF input, and combined RF output and bias input, as shown in the schematic diagrams in Figure 1. It has 4 leads including 2 ground leads; connecting both of them to external ground will minimize common path impedance for best RF performance. Multiple transistors are used in ERA-4, -5, and -6 to obtain high output power. This type of circuit is current- rather than voltage-controlled: for a range of current around the recommended value, the device voltage varies much less than in proportion to current. A constant-current DC source would be ideal for providing a stable operating point. By contrast, if a constant voltage DC source were used the current would vary widely with small changes in supply voltage, temperature change, and device-to-device variations. Therefore, use of the constant-voltage mode of operation requires caution.
A practical biasing configuration is shown in Figure 2. The RF current generated by the ERA in
response to an input signal develops an RF output voltage across an external load impedance.
Bias current is delivered from a voltage supply V Blocking capacitors are needed at the input and output ports. They should be of a type having low ESR (effective series resistance), and should have reactance low enough not to affect insertion loss or VSWR adversely at low frequency. The blocking capacitors must be free of parasitic (parallel) resonance up to the highest operating frequency. Use of a bypass capacitor at the connection to the DC supply is advised to prevent stray coupling to other signal processing components.
- In this circuit, DC blocking capacitors are added at the input port (pin number 1 on the packaged
amplifier) and at the output port (pin 3)
Bias current is given by the equation: I Table 1 lists the values of the bias resistor needed with several values of supply voltage based upon this equation, for each of the ERA models. It also lists the power dissipated by the bias resistor in the 12-volt case, as an example.
* Not recommended
The advisability of using an RF choke in series with R and the loss in power gain relative to not having the output loaded by R
Suppose, for example, that model ERA-4 is used with a 12-volt supply without a choke. From the above expression, the effect of the 108-ohm bias resistor (from Table 1) is found to be a 1.8 dB reduction in the gain of the amplifier. An RF choke should be chosen such that its reactance is at least 500 ohms (10 times the load impedance) at the lowest operating frequency. It must also be free of parasitic (series) resonance up to the highest operating frequency.
Increasing the resistor value reduces the variation in bias current making RF performance,
especially the 1-dB compression point, more constant. The device voltage V Typical voltage at several values of current is given in Table 2; the variation is expressed in the
right-hand column as the rate of change, V Device voltage decreases with increasing temperature as shown in Table 3; the average rate of
change is V
Based upon the above definitions for device-voltage variation, the variation of current with temperature can be derived: I where V The "" ratios are the coefficients from Tables 2 and 3, and V I Differentiating with respect to T: I To illustrate the effect of supply voltage, let us consider two examples for ERA-1, based upon
Tables 1, 2, and 3. For V I Over an operating temperature range of -45 to 85 degrees C, the total variation in current for this example will be 4.7 mA, which is more than 10% of the recommended value of current. The consequence is about 3.5 dB variation in output power at 1-dB compression. For the second example, try V
In addition to bias current stability, stability of power dissipation of the ERA device is favored
by using a high V P Taking the derivative of P
Let us see what happens to power dissipation in the case of ERA-5 between 25 and 85 degrees
C, for two values of supply voltage: one less than, and another greater than, twice the value of V For V Now, try V
Within the range of bias current listed in Table 2 for each ERA model, a value can be chosen to adjust gain and output power as needed. Recommended values are shown in bold type, and represent operating conditions for which junction temperature and reliability (MTTF) have been established. Higher currents than those listed could cause excessive junction temperature and premature failure. Substantially lower currents, while not degrading device reliability, could cause unpredictable RF performance because of non-optimum internal operating points.
An alternative method of biasing the ERA which allows use of lower supply voltage while
maintaining bias current stability and reducing power dissipation in the bias resistor is to use a
temperature compensating bias network in place of the single resistor R Commercially available chip thermistors such as the type noted above have a very high TCR (
temperature coefficient of resistance), +4500 ppm/°C, for resistance values in the range useful for
the ERA bias network, 51 - 510 ohms. The temperature coefficient needed for R We now derive the values of the network components. Let R be the resistance of the regular
resistor, and R Let k Let k Because the resistor and thermistor are in parallel they must satisfy: R Solving these equations yields: R = R Let us compare simple resistor biasing with the temperature compensating network for biasing an
ERA-6 with a 7.0 V supply. Let I Now, we compute the network component values needed to make the current 70 mA at 85°C as
well as at 25°C. At 85°C, given that V R = 20.0 x .27 (1 + .1285) ÷ (.27 - .1285) = 43.1 ohms R If the thermistors are available only in "5% values", sufficiently close compensation is obtained
by using R For different supply voltages and device operating points different resistor and thermistor values are needed, but the same concept and method can be used. The benefit is the ability to keep the device current constant over temperature, thereby avoiding increase in power dissipated in the amplifier and reduction in MTTF. We have not mentioned the resistance-temperature coefficient of the ordinary resistor in the bias network. The reason is that thick film chip resistors typically have a coefficients of ±100 ppm/°C. This is about 2% of the TCR of the thermistor, and does not influence the results significantly. A word of caution is due regarding the thermistor, however. Its temperature characteristic is controlled only at 25°C and at 75°C. The user should test actual circuit operation at other temperatures of interest. |