GaAs HBT technology has been successfully deployed for low to mid-power MMIC RF amplifiers in recent years. EiC Corp. pioneered the use of an InGaP emitter for RF power amplifiers and first introduced gain block products in 1998. InGaP/GaAs HBT devices have proven to provide better performance and be more reliable than their AlGaAs/GaAs HBT predecessors. Today, other semiconductor manufacturers have followed suit, and InGaP HBT products are superceding AlGaAs.

Recently, InGaP HBT capability has been taken to the next level with the introduction of a series of intermediate power amplifiers. Using the same process applied to already-offered lower power gain blocks, one-half-, one- and two-watt MMIC RF amplifiers are now available. The ECP05x series are 1/2 W, the ECP10x are 1 W and the ECP20x are 2 W amplifiers. Exceeding 1 W capability for a MMIC product is truly a milestone in the industry. Technical data referred to later in this article focuses on the ECP200 2 W MMIC amplifier.

Table 1 ECP-Series Intermediate Power Amplifier Characteristics
Model	P1dB* (dBm)	Gain* (dB)	OIP3* (dBm)	Frequency (MHz)
ECP050	27	15	45	1800 to 2300
ECP052	27	15	45	800 to 1000
ECP053	28	17	45	2100 to 2700
ECP100	31	12	47	100 to 2300
ECP103	31	12	47	2100 to 2700
ECP200	33	10	49	100 to 2300
ECP203	33	11	49	2100 to 2700
	*Typical values as stated on the data sheets

ECP Device Features

The ECP-series amplifiers are cost-effective, and feature a single-stage MMIC design in small outline plastic packaging with built-in ESD protection and the reliability benefits of InGaP HBT technology. The new amplifiers offer significant design flexibility by providing V_de up to 7 V, class A/AB selectivity, temperature compensation bias and power up/down sequencing. In addition, partial input matching circuitry is built on-chip. Table 1 lists the operating characteristics of the intermediate ECP-series.

Temperature Compensated Current Mirror Bias Circuit

Figure 1 shows a block diagram of the amplifier bias circuit. The amplifier is a single-stage design. The output matching circuit is provided off-chip and the collector current is provided through an RF choke of the output matching circuit.

The bias circuit is based on the current mirror principle and provides compensation over temperature for the bias current. V_bias requires a DC bias voltage, usually connected to V_cc, to provide the bias to the current mirror circuitry.

Figure 2 shows measured results for the class A bias condition for the 2 W, ECP200 amplifier over temperature. The RFout and V_bias are at 5 V, and the I_cc is measured against V_ref as V_ref is swept to 5 V. The measurement was performed over a -40° to +85°C ambient temperature range. The result clearly shows the bias current changes little over temperature. The current mirror bias circuit also provides power down capability.

As shown in the data plot, when V_ref is less than 2 V, the RF transistor is turned off. The power down capability allows the user greater flexibility in system design.

The ECP family features a single-stage design approach that provides flexibility for its users. ESD protection is included on-chip, protecting the circuit against 2000 V HBM (human body model). The ESD protection is not available with other discrete component solutions.

Partial Input Matching Circuit

As shown in the block diagram, the RF HBT is matched at the input, on-chip. For the ECP200 example, this input matching circuit is designed for the 2.14 GHz, W-CDMA base station transmitter frequency band. For lower frequency bands, the on-chip input matching circuit offers partial matching. Further matching made on the PCB will bring the input return loss to the desired level.

Class A or Class AB Bias

Table 2 compares the class A and class AB operation of the amplifier. The major trade-off is the output third-order intercept point (OIP3) with the power dissipation (efficiency) in a back-off state. For applications where OIP3 is not the top priority, class AB offers higher efficiency.

Probably the most interesting feature of the ECP-series to the designer is the flexibility of bias selection that allows the user to operate the amplifier in either class A or class AB, or anywhere in between.

The bias current of an ECP amplifier can be easily set (selected) in two ways: (1) by setting the V_ref voltage or (2) by adjusting the value of the external resistor on the V_ref pin. Method 1 is clearly shown in the I_cc/V_ref data plot.

I_cc versus V_ref is a linear relationship and the quiescent current can be set at any desired level, with little variation over temperature.

The user can change the external resistor value (Method 2) to modify the slope of the I_cc vs. V_ref response if needed for varying control capability. Method 2 can be understood by examining the diagram shown in Figure 3 . I_cq2 of the RF transistor Q2 is proportional to I_cq1 according to the ratio of transistor size, A2/A1. When I_cq1 (~I_ref) is increased, I_cq2 is increased accordingly. Since I_ref~(V_ref-2.5 V)/R, the quiescent current I_cq2 can be adjusted by the resistor value of R, while keeping V_ref at a fixed level.

By adding an external resistor on the PCB, the total value of R is increased. The reference current I_ref and quiescent current I_cq2 are reduced accordingly, changing the bias condition from class A to class AB.

The HBT breakdown voltage is over 20 V for BV_cbo and over 10 V for BV_ceo. Therefore, the amplifier can operate at higher than 5 V collector voltage. However, the ESD protection circuitry limits the collector voltage to approximately 12 V, which means V_cc is limited to 7 V to avoid the kick-on of the ESD protection.

When the device voltage is raised, the quiescent current must be reduced accordingly so the total DC bias power is constant. For example, the ECP200 amplifier is biased at 5 V and 800 mA. At 7 V, the current must be reduced to 570 mA, so the total power dissipation is maintained at 4 W.

The input matching is only slightly affected by the change of the collector voltage. However, the load impedance should be adjusted according to

R_L = (V_cc-V_k)/I_cc

Since V_cc is increased and I_cc is reduced, the load resistance will increase accordingly.

High Linearity

Many wireless communication systems require high linearity as a result of complicated signal modulation and multi-carrier applications.

As a result, the demand on OIP3 is ever increasing. A high OIP3 can be achieved by using either a higher output power transistor or a more linear transistor. The higher output power transistor requires more DC power consumption, which is undesirable. Therefore, a more linear transistor is preferred.

The ECP-series is a family of high linearity amplifiers. The linear figure of merit (OIP3-P1 dB) for EiC's InGaP HBT process has historically been from 15 to 17 dB.

This high figure of merit has been maintained for the higher power ECP-series devices and provides outstanding performance to satisfy many infrastructure applications.

Figure 4 shows the linearity results using a W-CDMA signal for a base station. Both class A and class AB (50 percent class A quiescent current) were tested. The adjacent channel leakage ratio (ACLR) for both bias conditions is similar at the same output power, but the efficiency is higher for the class AB mode.

Figure 5 summarizes the two-tone test results. The third-order intermodulation (IM3) in class A is better than class AB at lower power levels, until both reach 26 dBm single carrier level (SCL). Twenty-six dBm SCL corresponds to 32 dBm peak envelope power (PEP); therefore, above this power level, waveform clipping starts and the IM3 in both bias conditions becomes similar.

It can be seen from the reported test results that the ECP200 intermediate power amplifier, as well as its lower power bretheren, demonstrates outstanding performance. Now the RF designer has a truly cost-effective option for higher power amplifier requirements, a result of the company's leading edge InGaP/GaAs HBT technology. The devices can also be configured in parallel to meet even higher power out requirements, well under the cost of existing solutions.

EiC Corp., Fremont, CA (510) 979 8953, www.eiccorp.com. Circle No. 302